Quest for an Electric Porsche Boxster.
Today I will update the status of the Porsche Boxster. My personal and professional life have had many detours in recent months, so the Porsche has not been as much a priority. However, I have been driving the car lately, and have many new experiences to share, so here we go!
Batteries:
The configuration using K2 batteries in blades without forced air cooling has been a major disappointment. The batteries have degraded in capacity over time, reducing range and power. The car now goes about 25-30 miles if driven VERY conservatively. Efficiency is about 315Wh/m at best. The power is less now, putting out about 650A max, which is still enough to comfortably drive the car, but it's no racer.
When we designed the car, battery options were limited. Now, if we were choosing batteries again, we would strongly consider the CALB 130Ah cells that are relatively small and put out pretty good power. I think enough cells could fit in the car to get over 240V and over 100 miles range. A123 pouch cells are becoming available, with more potential for a supercar, but integrating them into a module is still a relative pain compared to prismatics. I have heard positive things about high power Headway cells, but CALBs would be more practical IMO, and the car would be powerful enough.
Motor:
Warp 11 has been stable since fan blew apart. We had trouble with the spindle rubbing on the adapter plate, but got that fixed by drilling into the motor crankshaft. Now it spins smooth.
Controller:
Most reliable part of car. The car would have been fine with a Z1K, and even the newer Soliton1 or Warp controllers. I can't wait to see how it will run with a more powerful pack that can get closer to 2000A.
Charger:
Sent it back and they adjusted the thermistor and added a fan to the liquid cooling. So now we get 42A for about 15 min before it throttles back to around 35A for rest fo charge due to overheating. We may send it back again to add another fan which should do it. Since the car doesn't have much range, the faster you can charge it the better.
Evision:
After discussions with Victor, he finally decided to accept the return of the eVision 1, and sent us a new eVision 2. We worked hard redesigning the install, and once done, had a similar problem... inacurrate amp measurements. This time it was also crashing sporadically. The charging shunt which is new on eVision 2 worked fine. The shunt board is mounted on side of front battery pack. So we removed the whole thing again, added some foam for support of the back of the circuit board, and rearranged the big high voltage wires nearby. To our surprise, it is working MUCH BETTER now. We just finished, so still evaluating, but I LIKE IT! Especially with a short range, the eVision allows me to keep very good track of state of charge between charges. Right now, I think the problem was installation, and not the eVision 1 since it was working until Mark took the front string out to do some work. I think he must have arranged the high voltage wires in such a way that it was causing interference. Just a guess. I would recommend installing the eVision shunt board away from crossed high voltage wires. Also, the eVision 2 is MUCH easier to see driving in the day.
BMS:
Had a bolt drop on one of the boards in the rear battery string. We were out of working boards, so the guys tried to build one from hand and finally succeeded. We decided to buy a professional run of 12 boards for back up which cost over $500. One of the boards is not giving proper temperature readings and will be replaced. Also, the front string is crashing periodically, probably related to a bad BMS board. We are still working of troubleshooting that one.
Suspension:
Seems loose in hard turns, so will need to get it aligned again. Probably messed it up taking the motor/transmission out.
Brakes:
Had problem with rear brakes on right making loud noises. Ended up needing to replace all the brakes and rotors, which on a Porsche ended up costing $1200.
Convertible top:
Had it replace with a glass rear window and then had the car ceramic tinted. Looks nice. Had to replace convertible top transmissions and cables. Bought them on eBay for a couple hundred dollars and installed them myself. Works fine now.
In conclusion, I think the Boxster makes a great conversion. Now that we have learned the pitfalls, if we can install a better battery pack the car would be outstanding. Even so, the car is great fun to drive, and I don't regret the project overall. I will probably drive the current battery pack till it is getting under 20 miles range, as that is close to round trip distance to work. Then we will do the upgrade, and probably battery prices will be much less and the loss less painful. I am also going to upgrade the speakers as Porsche was cheap on the sound system. My old Subaru had a better sound system. Good luck to all you guys out there trying to convert Porsches to electric!
Saturday, January 21, 2012
Friday, May 6, 2011
Minus three blades, no loss of range
I did a range test today without trying to be very efficient. I went 34 miles before several of the cells started getting low. There are still marked variances in the cells. Awaiting bench tests on the three blades recently removed.
Quest for an Electric Porsche Boxster.
Wednesday, April 27, 2011
K2 26650EV Battery Pack Testing
Quest for an Electric Porsche Boxster.
After 2 months of Chris' hard work reprogramming the BMS software to automatically shut down the charger, we finally have the car back for more range tests. We also have a very nice graphical interface that is much easier to look at than the raw data! As requested by K2, we overcharged the pack to 3.8v for about 3 days. The idea is to try and force more capacity into the cells since they are supposed to hold more. Well it hasn't worked. Today I ran a second range test with similar results. The same damaged cell in the front pack comes up first. Then after turning off the front pack, I am able to squeeze out a few more miles, but nothing close to the expected 55-60mile range predicted from K2 specs. The graph shows all 3 strings: 64 cells on the left is the front pack, 64 in the middle, the mid packs, and 64 on the right the rear pack. The first graph shows the maximum range before limitation from the previously damaged cell in the front pack.
I turned the front string off and eeked out another 4.6 miles.
Here is the second graph at the end of the range test at 38.7 miles:
This was mixed driving, trying to be at least somewhat efficient. I suspect if I did all highway I would have had a couple more miles, perhaps 45 miles if the damaged cell in the front pack was replaced. 45 miles is the best I have ever done with this pack.
So far, these cells are not improving with 3 days of overcharging at 3.8v. None of the blades in the mid and rear pack stand out as being bad compared to the rest, and there are a few that seem a bit higher capacity than the rest. But I doubt much more. We should pull the front bad blade, one of the lower and one of the highest blades from the mid and rear packs. Then test them all for capacity to show K2 and see what they will do.
Analyzing the graphs indicate 2-41 and 3-24 did come up as the next lowest cells again in the mid and rear packs... although 19 cells were within close proximity. It is also interesting that there is a disparity between the two mid packs. One has a lid on it and the other doesn't. The front and rear packs do not have lids. Still I think the difference is probably marginal in total capacity. I am removing the lids as the packs get hot when the car is driven a bit hard, especially if its hot outside. We want to keep the temps within spec, no greater than 60 degrees C. So, we will need to put some fans on the battery boxes at some point.
After 2 months of Chris' hard work reprogramming the BMS software to automatically shut down the charger, we finally have the car back for more range tests. We also have a very nice graphical interface that is much easier to look at than the raw data! As requested by K2, we overcharged the pack to 3.8v for about 3 days. The idea is to try and force more capacity into the cells since they are supposed to hold more. Well it hasn't worked. Today I ran a second range test with similar results. The same damaged cell in the front pack comes up first. Then after turning off the front pack, I am able to squeeze out a few more miles, but nothing close to the expected 55-60mile range predicted from K2 specs. The graph shows all 3 strings: 64 cells on the left is the front pack, 64 in the middle, the mid packs, and 64 on the right the rear pack. The first graph shows the maximum range before limitation from the previously damaged cell in the front pack.
I turned the front string off and eeked out another 4.6 miles.
Here is the second graph at the end of the range test at 38.7 miles:
This was mixed driving, trying to be at least somewhat efficient. I suspect if I did all highway I would have had a couple more miles, perhaps 45 miles if the damaged cell in the front pack was replaced. 45 miles is the best I have ever done with this pack.
So far, these cells are not improving with 3 days of overcharging at 3.8v. None of the blades in the mid and rear pack stand out as being bad compared to the rest, and there are a few that seem a bit higher capacity than the rest. But I doubt much more. We should pull the front bad blade, one of the lower and one of the highest blades from the mid and rear packs. Then test them all for capacity to show K2 and see what they will do.
Analyzing the graphs indicate 2-41 and 3-24 did come up as the next lowest cells again in the mid and rear packs... although 19 cells were within close proximity. It is also interesting that there is a disparity between the two mid packs. One has a lid on it and the other doesn't. The front and rear packs do not have lids. Still I think the difference is probably marginal in total capacity. I am removing the lids as the packs get hot when the car is driven a bit hard, especially if its hot outside. We want to keep the temps within spec, no greater than 60 degrees C. So, we will need to put some fans on the battery boxes at some point.
Tuesday, March 1, 2011
Wednesday, February 23, 2011
Bottom Misbalancing Act III
Quest for an Electric Porsche Boxster.
I bottom balanced again, this time down to 2.5v +/- 0.05v with shunt balancers. About 68Ah went into pack before blade 42-1 became full which I carefully allowed up to 3.8v at 1A on charger. The closest next cell was at 3.3v. Following is at the end of test drive conservatively averaging about 65mph, mostly freeway:
01 2866 037 2869 037 2862 037 2862 037
02 2860 038 2882 038 2855 038 2852 038
03 2705 041 2584 041 2688 041 2505 041
04 2697 045 2732 045 2694 045 2519 045
05 2677 047 2653 047 2714 047 2694 047
06 2913 043 2931 043 2926 043 2865 043
07 2463 046 2628 046 2633 046 2623 046
08 2635 044 2613 044 2634 044 2685 044
09 2615 042 2635 042 2664 042 2567 042
10 2739 050 2734 050 2648 050 2669 050
11 2710 048 2737 048 2699 048 2691 048
12 2641 048 2684 048 2678 048 2699 048
13 2739 048 2730 048 2724 048 2616 048
14 2672 047 2699 047 2672 047 2687 047
15 2723 047 2707 047 2665 047 2672 047
16 2656 045 2657 045 2648 045 2630 045
17 2606 044 2623 044 2649 044 2639 044
18 2657 051 2650 051 2642 051 2623 051
19 2852 044 2863 044 2849 044 2847 044
20 2646 047 2649 047 2667 047 2695 047
21 2862 044 2884 044 2878 044 2824 044
22 2851 044 2866 044 2857 044 2860 044
23 2634 051 2669 051 2667 051 2646 051
24 2836 043 2864 043 2814 043 2802 043
25 2663 047 2662 047 2703 047 2686 047
26 2619 049 2643 049 2590 049 2646 049
27 2656 053 2635 053 2698 053 2628 053
28 2667 053 2634 053 2688 053 2695 053
29 2577 051 2540 051 2543 051 2600 051
30 2593 049 2647 049 2671 049 2664 049
31 2651 051 2554 051 2657 051 2667 051
32 2636 042 2598 042 2623 042 2588 042
33 2640 044 2675 044 2713 044 2700 044
34 2663 049 2702 049 2684 049 2686 049
35 2660 051 2502 051 2618 051 2676 051
36 2571 053 2583 053 2647 053 2628 053
37 2615 049 2657 049 2656 049 2669 049
38 2876 047 2882 047 2894 047 2893 047
39 2595 049 2578 049 2612 049 2588 049
40 2917 048 2927 048 2923 048 2899 048
41 2616 049 2663 049 2672 049 2695 049
42 2506 051 2706 051 2740 051 2745 051
43 2573 051 2566 051 2613 051 2597 051
44 2672 049 2661 049 2663 049 2681 049
45 2874 046 2881 046 2908 046 2882 046
46 2664 048 2653 048 2705 048 2700 048
47 2625 048 2617 048 2599 048 2640 048
48 2644 041 2639 041 2667 041 2667 041
Max Voltage: 2931 (62)
Min Voltage: 2463 {71)
Max Temp: 53
Min Temp: 37
under load coming into driveway
66.4Ah 41.3m, 1.6 Ah/mile
after 5min, increased to 2.8v drove around block again:
01 2912 038 2917 038 2914 038 2910 038
02 2897 041 2906 041 2910 041 2912 041
03 2778 042 2754 042 2778 042 2731 042
04 2801 047 2811 047 2796 047 2750 047
05 2759 050 2761 050 2787 050 2783 050
06 2921 044 2930 044 2932 044 2910 044
07 2745 047 2762 047 2774 047 2773 047
08 2761 045 2750 045 2767 045 2787 045
09 2743 044 2750 044 2766 044 2726 044
10 2802 052 2804 052 2770 052 2786 052
11 2790 049 2803 049 2787 049 2784 049
12 2764 050 2783 050 2779 050 2787 050
13 2800 050 2794 050 2795 050 2758 050
14 2773 049 2787 049 2770 049 2770 049
15 2791 049 2789 049 2758 049 2774 049
16 2757 047 2757 047 2764 047 2756 047
17 2744 044 2760 044 2768 044 2762 044
18 2782 052 2777 052 2773 052 2777 052
19 2906 046 2908 046 2906 046 2904 046
20 2749 048 2754 048 2748 048 2756 048
21 2906 046 2910 046 2912 046 2905 046
22 2903 045 2914 045 2909 045 2908 045
23 2753 052 2771 052 2767 052 2765 052
24 2908 044 2912 044 2907 044 2894 044
25 2765 049 2759 049 2760 049 2759 049
26 2750 050 2748 050 2714 050 2748 050
27 2767 054 2750 054 2769 054 2746 054
28 2770 055 2746 055 2763 055 2761 055
29 2737 053 2711 053 2708 053 2735 053
30 2746 051 2775 051 2763 051 2774 051
31 2760 051 2756 051 2752 051 2761 051
32 2755 045 2764 045 2742 045 2744 045
33 2760 045 2773 045 2784 045 2774 045
34 2790 050 2799 050 2783 050 2788 050
35 2796 052 2579 052 2783 052 2790 052
36 2736 055 2735 055 2751 055 2740 055
37 2761 050 2780 050 2767 050 2787 050
38 2924 049 2921 049 2925 049 2924 049
39 2755 050 2742 050 2753 050 2727 050
40 2920 049 2925 049 2918 049 2909 049
41 2762 051 2778 051 2764 051 2774 051
42 2671 053 2784 053 2790 053 2794 053
43 2738 053 2738 053 2739 053 2740 053
44 2753 050 2733 050 2736 050 2757 050
45 2924 048 2929 048 2926 048 2920 048
46 2774 049 2771 049 2785 049 2785 049
47 2757 049 2752 049 2734 049 2755 049
48 2780 040 2792 040 2787 040 2788 040
Max Voltage: 2932 (63)
Min Voltage: 2579 {352)
Max Temp: 55
Min Temp: 38
68.2Ah, 42m, 1.62Ah/mile
I squeezed out another 3Ah on the last test drive, but went a bit lower on average end of discharge voltages. The overdischarge two weeks ago cost me 6-7Ah total capacity and about 4 miles range compared to best range test before overdischarge. I believe the loss is mainly from 42-1 which was low to begin with. I don't think blades 5 or 27 are seriously damaged. But they are lower than the average blade in the pack which is about 15% less than it should be after accounting for perhaps 4% per year loss in storage with K2 cells. I am not sure other LiFePo4 cells have the same problem as most are not documenting it.
I don't think bottom balancing will work with this many cells and without similar capacity cells, so I am going back to top balancing for now. I will recheck conservative range after top balancing, and I think it is about the same since I was getting about 65Ah capacity after the overdischarge event.
I bottom balanced again, this time down to 2.5v +/- 0.05v with shunt balancers. About 68Ah went into pack before blade 42-1 became full which I carefully allowed up to 3.8v at 1A on charger. The closest next cell was at 3.3v. Following is at the end of test drive conservatively averaging about 65mph, mostly freeway:
01 2866 037 2869 037 2862 037 2862 037
02 2860 038 2882 038 2855 038 2852 038
03 2705 041 2584 041 2688 041 2505 041
04 2697 045 2732 045 2694 045 2519 045
05 2677 047 2653 047 2714 047 2694 047
06 2913 043 2931 043 2926 043 2865 043
07 2463 046 2628 046 2633 046 2623 046
08 2635 044 2613 044 2634 044 2685 044
09 2615 042 2635 042 2664 042 2567 042
10 2739 050 2734 050 2648 050 2669 050
11 2710 048 2737 048 2699 048 2691 048
12 2641 048 2684 048 2678 048 2699 048
13 2739 048 2730 048 2724 048 2616 048
14 2672 047 2699 047 2672 047 2687 047
15 2723 047 2707 047 2665 047 2672 047
16 2656 045 2657 045 2648 045 2630 045
17 2606 044 2623 044 2649 044 2639 044
18 2657 051 2650 051 2642 051 2623 051
19 2852 044 2863 044 2849 044 2847 044
20 2646 047 2649 047 2667 047 2695 047
21 2862 044 2884 044 2878 044 2824 044
22 2851 044 2866 044 2857 044 2860 044
23 2634 051 2669 051 2667 051 2646 051
24 2836 043 2864 043 2814 043 2802 043
25 2663 047 2662 047 2703 047 2686 047
26 2619 049 2643 049 2590 049 2646 049
27 2656 053 2635 053 2698 053 2628 053
28 2667 053 2634 053 2688 053 2695 053
29 2577 051 2540 051 2543 051 2600 051
30 2593 049 2647 049 2671 049 2664 049
31 2651 051 2554 051 2657 051 2667 051
32 2636 042 2598 042 2623 042 2588 042
33 2640 044 2675 044 2713 044 2700 044
34 2663 049 2702 049 2684 049 2686 049
35 2660 051 2502 051 2618 051 2676 051
36 2571 053 2583 053 2647 053 2628 053
37 2615 049 2657 049 2656 049 2669 049
38 2876 047 2882 047 2894 047 2893 047
39 2595 049 2578 049 2612 049 2588 049
40 2917 048 2927 048 2923 048 2899 048
41 2616 049 2663 049 2672 049 2695 049
42 2506 051 2706 051 2740 051 2745 051
43 2573 051 2566 051 2613 051 2597 051
44 2672 049 2661 049 2663 049 2681 049
45 2874 046 2881 046 2908 046 2882 046
46 2664 048 2653 048 2705 048 2700 048
47 2625 048 2617 048 2599 048 2640 048
48 2644 041 2639 041 2667 041 2667 041
Max Voltage: 2931 (62)
Min Voltage: 2463 {71)
Max Temp: 53
Min Temp: 37
under load coming into driveway
66.4Ah 41.3m, 1.6 Ah/mile
after 5min, increased to 2.8v drove around block again:
01 2912 038 2917 038 2914 038 2910 038
02 2897 041 2906 041 2910 041 2912 041
03 2778 042 2754 042 2778 042 2731 042
04 2801 047 2811 047 2796 047 2750 047
05 2759 050 2761 050 2787 050 2783 050
06 2921 044 2930 044 2932 044 2910 044
07 2745 047 2762 047 2774 047 2773 047
08 2761 045 2750 045 2767 045 2787 045
09 2743 044 2750 044 2766 044 2726 044
10 2802 052 2804 052 2770 052 2786 052
11 2790 049 2803 049 2787 049 2784 049
12 2764 050 2783 050 2779 050 2787 050
13 2800 050 2794 050 2795 050 2758 050
14 2773 049 2787 049 2770 049 2770 049
15 2791 049 2789 049 2758 049 2774 049
16 2757 047 2757 047 2764 047 2756 047
17 2744 044 2760 044 2768 044 2762 044
18 2782 052 2777 052 2773 052 2777 052
19 2906 046 2908 046 2906 046 2904 046
20 2749 048 2754 048 2748 048 2756 048
21 2906 046 2910 046 2912 046 2905 046
22 2903 045 2914 045 2909 045 2908 045
23 2753 052 2771 052 2767 052 2765 052
24 2908 044 2912 044 2907 044 2894 044
25 2765 049 2759 049 2760 049 2759 049
26 2750 050 2748 050 2714 050 2748 050
27 2767 054 2750 054 2769 054 2746 054
28 2770 055 2746 055 2763 055 2761 055
29 2737 053 2711 053 2708 053 2735 053
30 2746 051 2775 051 2763 051 2774 051
31 2760 051 2756 051 2752 051 2761 051
32 2755 045 2764 045 2742 045 2744 045
33 2760 045 2773 045 2784 045 2774 045
34 2790 050 2799 050 2783 050 2788 050
35 2796 052 2579 052 2783 052 2790 052
36 2736 055 2735 055 2751 055 2740 055
37 2761 050 2780 050 2767 050 2787 050
38 2924 049 2921 049 2925 049 2924 049
39 2755 050 2742 050 2753 050 2727 050
40 2920 049 2925 049 2918 049 2909 049
41 2762 051 2778 051 2764 051 2774 051
42 2671 053 2784 053 2790 053 2794 053
43 2738 053 2738 053 2739 053 2740 053
44 2753 050 2733 050 2736 050 2757 050
45 2924 048 2929 048 2926 048 2920 048
46 2774 049 2771 049 2785 049 2785 049
47 2757 049 2752 049 2734 049 2755 049
48 2780 040 2792 040 2787 040 2788 040
Max Voltage: 2932 (63)
Min Voltage: 2579 {352)
Max Temp: 55
Min Temp: 38
68.2Ah, 42m, 1.62Ah/mile
I squeezed out another 3Ah on the last test drive, but went a bit lower on average end of discharge voltages. The overdischarge two weeks ago cost me 6-7Ah total capacity and about 4 miles range compared to best range test before overdischarge. I believe the loss is mainly from 42-1 which was low to begin with. I don't think blades 5 or 27 are seriously damaged. But they are lower than the average blade in the pack which is about 15% less than it should be after accounting for perhaps 4% per year loss in storage with K2 cells. I am not sure other LiFePo4 cells have the same problem as most are not documenting it.
I don't think bottom balancing will work with this many cells and without similar capacity cells, so I am going back to top balancing for now. I will recheck conservative range after top balancing, and I think it is about the same since I was getting about 65Ah capacity after the overdischarge event.
Tuesday, February 22, 2011
Bottom Misbalancing Act II
Quest for an Electric Porsche Boxster.
I was trying to bottom balance again after 35-2 was early reaching 2.5v after driving this evening. The pack took a few hours before 42-1 shunted down to the level of 35-2. I turned off the BMS ssh data stream for a few minutes to take a break from the warning texts I receive when any cell goes below 2.5v. When I checked again 15 minutes later 35-2 had magically jumped up to >2.8v while 42-1 stayed down at <2.5v. I had not turned off the breakers to the strings. The charger is off and the car unplugged. I double checked 35-2 manually and it was at 2.8v and now shunting. An hour ago it was manually 2.5v and not shunting. Perplexing. Now 35-2 is the max voltage cell in the pack! The connections look fine and manual meter matches the BMS board.
01 2814 028 2844 028 2821 028 2823 028
02 2848 028 2869 028 2850 028 2853 028
03 2777 022 2812 022 2791 022 2807 022
04 2777 032 2812 032 2793 032 2794 032
05 2746 033 2764 033 2756 033 2770 033
06 2794 033 2839 033 2814 033 2810 033
07 2812 032 2828 032 2826 032 2827 032
08 2772 032 2801 032 2781 032 2772 032
09 2783 030 2812 030 2788 030 2790 030
10 2771 037 2797 037 2788 037 2790 037
11 2751 035 2773 035 2750 035 2736 035
12 2746 036 2779 036 2731 036 2776 036
13 2755 037 2775 037 2767 037 2760 037
14 2720 037 2761 037 2729 037 2748 037
15 2783 034 2802 034 2778 034 2795 034
16 2811 035 2849 035 2822 035 2835 035
17 2802 033 2823 033 2802 033 2811 033
18 2772 041 2787 041 2771 041 2785 041
19 2798 034 2824 034 2801 034 2809 034
20 2741 042 2771 042 2752 042 2751 042
21 2831 035 2864 035 2856 035 2844 035
22 2818 035 2860 035 2835 035 2834 035
23 2716 041 2772 041 2774 041 2787 041
24 2861 032 2887 032 2878 032 2878 032
25 2789 036 2813 036 2794 036 2795 036
26 2764 037 2786 037 2779 037 2780 037
27 2773 040 2796 040 2777 040 2780 040
28 2709 042 2730 042 2714 042 2732 042
29 2735 040 2759 040 2738 040 2754 040
30 2752 035 2768 035 2765 035 2772 035
31 2723 037 2749 037 2724 037 2741 037
32 2763 033 2792 033 2779 033 2787 033
33 2571 031 2641 031 2636 031 2655 031
34 2783 035 2812 035 2796 035 2800 035
35 2785 036 2914 036 2789 036 2807 036
36 2796 038 2802 038 2796 038 2805 038
37 2771 037 2797 037 2786 037 2801 037
38 2796 038 2861 038 2854 038 2850 038
39 2771 038 2800 038 2777 038 2794 038
40 2816 039 2848 039 2813 039 2819 039
41 2777 038 2796 038 2776 038 2791 038
42 2490 037 2798 037 2787 037 2797 037
43 2767 037 2800 037 2791 037 2789 037
44 2775 037 2791 037 2782 037 2795 037
45 2835 037 2882 037 2845 037 2857 037
46 2794 035 2815 035 2804 035 2802 035
47 2765 033 2792 033 2766 033 2770 033
48 2788 029 2801 029 2782 029 2787 029
Max Voltage: 2914 (352)
Min Voltage: 2490 {421)
Max Temp: 42
Min Temp: 22
I was trying to bottom balance again after 35-2 was early reaching 2.5v after driving this evening. The pack took a few hours before 42-1 shunted down to the level of 35-2. I turned off the BMS ssh data stream for a few minutes to take a break from the warning texts I receive when any cell goes below 2.5v. When I checked again 15 minutes later 35-2 had magically jumped up to >2.8v while 42-1 stayed down at <2.5v. I had not turned off the breakers to the strings. The charger is off and the car unplugged. I double checked 35-2 manually and it was at 2.8v and now shunting. An hour ago it was manually 2.5v and not shunting. Perplexing. Now 35-2 is the max voltage cell in the pack! The connections look fine and manual meter matches the BMS board.
01 2814 028 2844 028 2821 028 2823 028
02 2848 028 2869 028 2850 028 2853 028
03 2777 022 2812 022 2791 022 2807 022
04 2777 032 2812 032 2793 032 2794 032
05 2746 033 2764 033 2756 033 2770 033
06 2794 033 2839 033 2814 033 2810 033
07 2812 032 2828 032 2826 032 2827 032
08 2772 032 2801 032 2781 032 2772 032
09 2783 030 2812 030 2788 030 2790 030
10 2771 037 2797 037 2788 037 2790 037
11 2751 035 2773 035 2750 035 2736 035
12 2746 036 2779 036 2731 036 2776 036
13 2755 037 2775 037 2767 037 2760 037
14 2720 037 2761 037 2729 037 2748 037
15 2783 034 2802 034 2778 034 2795 034
16 2811 035 2849 035 2822 035 2835 035
17 2802 033 2823 033 2802 033 2811 033
18 2772 041 2787 041 2771 041 2785 041
19 2798 034 2824 034 2801 034 2809 034
20 2741 042 2771 042 2752 042 2751 042
21 2831 035 2864 035 2856 035 2844 035
22 2818 035 2860 035 2835 035 2834 035
23 2716 041 2772 041 2774 041 2787 041
24 2861 032 2887 032 2878 032 2878 032
25 2789 036 2813 036 2794 036 2795 036
26 2764 037 2786 037 2779 037 2780 037
27 2773 040 2796 040 2777 040 2780 040
28 2709 042 2730 042 2714 042 2732 042
29 2735 040 2759 040 2738 040 2754 040
30 2752 035 2768 035 2765 035 2772 035
31 2723 037 2749 037 2724 037 2741 037
32 2763 033 2792 033 2779 033 2787 033
33 2571 031 2641 031 2636 031 2655 031
34 2783 035 2812 035 2796 035 2800 035
35 2785 036 2914 036 2789 036 2807 036
36 2796 038 2802 038 2796 038 2805 038
37 2771 037 2797 037 2786 037 2801 037
38 2796 038 2861 038 2854 038 2850 038
39 2771 038 2800 038 2777 038 2794 038
40 2816 039 2848 039 2813 039 2819 039
41 2777 038 2796 038 2776 038 2791 038
42 2490 037 2798 037 2787 037 2797 037
43 2767 037 2800 037 2791 037 2789 037
44 2775 037 2791 037 2782 037 2795 037
45 2835 037 2882 037 2845 037 2857 037
46 2794 035 2815 035 2804 035 2802 035
47 2765 033 2792 033 2766 033 2770 033
48 2788 029 2801 029 2782 029 2787 029
Max Voltage: 2914 (352)
Min Voltage: 2490 {421)
Max Temp: 42
Min Temp: 22
Bottom Misbalancing Act
Quest for an Electric Porsche Boxster.
I am not sure what to say... I bottom balanced, and it seemed to work the first full charge and discharge. But the second full charge and discharge seems to indicate drift. And this time it is blade 35-2 which is much lower than the rest! I don't get it. It is in the SAME string as 42-1, and now 35 is the lowest cell in the string and pack by far. 35 hasn't even really come up before as a low capacity cell. I manually checked the voltage and it was true, 2.5v. I am going to try and bottom balance one more time, this time down to 2.6v to see if the drift occurs again. If it does, I don't think these cells discharge or charge consistently, perhaps due to wide variations of internal resistance and capacities. I like the power output, but the capacity and consistency of these cells has been a real pain.
01 3055 029 3066 029 3064 029 3069 029
02 3063 031 3071 031 3064 031 3072 031
03 2996 022 2986 022 3001 022 3000 022
04 3001 034 3011 034 3011 034 3015 034
05 2979 035 2979 035 2982 035 2995 035
06 3046 034 3068 034 3057 034 3059 034
07 2958 034 2991 034 3000 034 3007 034
08 2987 034 2992 034 2996 034 3005 034
09 2985 032 2990 032 2995 032 2985 032
10 2999 039 3006 039 2998 039 3007 039
11 2984 037 2993 037 2981 037 2985 037
12 2977 037 2985 037 2968 037 2996 037
13 2991 037 2990 037 2989 037 2986 037
14 2968 037 2976 037 2970 037 2983 037
15 2991 036 2994 036 2982 036 2999 036
16 2996 036 3009 036 3004 036 3005 036
17 2992 032 2998 032 3005 032 3007 032
18 2993 038 2991 038 2987 038 3000 038
19 3042 033 3047 033 3047 033 3055 033
20 2963 035 2968 035 2967 035 2981 035
21 3050 034 3062 034 3057 034 3060 034
22 3045 033 3058 033 3048 033 3054 033
23 2977 037 2992 037 2990 037 2997 037
24 3059 032 3075 032 3067 032 3068 032
25 2993 036 2992 036 2986 036 2996 036
26 2977 036 2970 036 2964 036 2982 036
27 2982 039 2975 039 2982 039 2980 039
28 2970 040 2957 040 2966 040 2984 040
29 2962 037 2952 037 2945 037 2967 037
30 2974 035 2982 035 2979 035 2996 035
31 2962 036 2963 036 2959 036 2983 036
32 2983 031 2991 031 2977 031 2990 031
33 2938 033 2946 033 2955 033 2960 033
34 2992 037 3018 037 3013 037 3023 037
35 3008 038 2504 038 3007 038 3014 038
36 2987 041 2988 041 2987 041 2994 041
37 2993 038 2998 038 2995 038 3014 038
38 3064 038 3070 038 3077 038 3079 038
39 2995 038 2995 038 2997 038 3010 038
40 3057 039 3070 039 3055 039 3058 039
41 2987 038 2992 038 2982 038 2995 038
42 2775 039 3016 039 3005 039 3015 039
43 2991 039 2990 039 2994 039 3003 039
44 2991 038 2987 038 2987 038 3001 038
45 3063 037 3077 037 3063 037 3064 037
46 3010 036 3003 036 3008 036 3013 036
47 2993 036 3000 036 2995 036 3003 036
48 3008 031 3010 031 3012 031 3019 031
Max Voltage: 3079 (384)
Min Voltage: 2504 {352)
Max Temp: 41
Min Temp: 22
I am not sure what to say... I bottom balanced, and it seemed to work the first full charge and discharge. But the second full charge and discharge seems to indicate drift. And this time it is blade 35-2 which is much lower than the rest! I don't get it. It is in the SAME string as 42-1, and now 35 is the lowest cell in the string and pack by far. 35 hasn't even really come up before as a low capacity cell. I manually checked the voltage and it was true, 2.5v. I am going to try and bottom balance one more time, this time down to 2.6v to see if the drift occurs again. If it does, I don't think these cells discharge or charge consistently, perhaps due to wide variations of internal resistance and capacities. I like the power output, but the capacity and consistency of these cells has been a real pain.
01 3055 029 3066 029 3064 029 3069 029
02 3063 031 3071 031 3064 031 3072 031
03 2996 022 2986 022 3001 022 3000 022
04 3001 034 3011 034 3011 034 3015 034
05 2979 035 2979 035 2982 035 2995 035
06 3046 034 3068 034 3057 034 3059 034
07 2958 034 2991 034 3000 034 3007 034
08 2987 034 2992 034 2996 034 3005 034
09 2985 032 2990 032 2995 032 2985 032
10 2999 039 3006 039 2998 039 3007 039
11 2984 037 2993 037 2981 037 2985 037
12 2977 037 2985 037 2968 037 2996 037
13 2991 037 2990 037 2989 037 2986 037
14 2968 037 2976 037 2970 037 2983 037
15 2991 036 2994 036 2982 036 2999 036
16 2996 036 3009 036 3004 036 3005 036
17 2992 032 2998 032 3005 032 3007 032
18 2993 038 2991 038 2987 038 3000 038
19 3042 033 3047 033 3047 033 3055 033
20 2963 035 2968 035 2967 035 2981 035
21 3050 034 3062 034 3057 034 3060 034
22 3045 033 3058 033 3048 033 3054 033
23 2977 037 2992 037 2990 037 2997 037
24 3059 032 3075 032 3067 032 3068 032
25 2993 036 2992 036 2986 036 2996 036
26 2977 036 2970 036 2964 036 2982 036
27 2982 039 2975 039 2982 039 2980 039
28 2970 040 2957 040 2966 040 2984 040
29 2962 037 2952 037 2945 037 2967 037
30 2974 035 2982 035 2979 035 2996 035
31 2962 036 2963 036 2959 036 2983 036
32 2983 031 2991 031 2977 031 2990 031
33 2938 033 2946 033 2955 033 2960 033
34 2992 037 3018 037 3013 037 3023 037
35 3008 038 2504 038 3007 038 3014 038
36 2987 041 2988 041 2987 041 2994 041
37 2993 038 2998 038 2995 038 3014 038
38 3064 038 3070 038 3077 038 3079 038
39 2995 038 2995 038 2997 038 3010 038
40 3057 039 3070 039 3055 039 3058 039
41 2987 038 2992 038 2982 038 2995 038
42 2775 039 3016 039 3005 039 3015 039
43 2991 039 2990 039 2994 039 3003 039
44 2991 038 2987 038 2987 038 3001 038
45 3063 037 3077 037 3063 037 3064 037
46 3010 036 3003 036 3008 036 3013 036
47 2993 036 3000 036 2995 036 3003 036
48 3008 031 3010 031 3012 031 3019 031
Max Voltage: 3079 (384)
Min Voltage: 2504 {352)
Max Temp: 41
Min Temp: 22
Sunday, February 20, 2011
Bottom Balancing Act I
Quest for an Electric Porsche Boxster.
As with most things in life, I often have to learn things the hard way. The last deep discharge probably did damage the weakest blade, 42. I don't seem to be getting as much capacity out of the top balanced pack, and I am at risk of running the weakest blades down to reversal every time I drive the car too far. So after some soul searching and a little help from my friends Jack and Tim, I decided to buck from my team at Revolt, and bottom balance. Thankfully, bottom balancing with a BMS was not so painful to accomplish. I drove the car to help discharge the batteries, then I reset the BMS shunts at 2.75v. It took another 36 hours for the top balanced cells to shunt down, and then I gradually charged up the lower cells to 2.75 +/- 0.02v. I then reset the shunts to 3.9v and charged up the pack, manually shutting off the charger when 42-1 got to 3.7v taking in about 71.3Ah.
I drove the car and it was much more predictable discharging and none of the cells went much below 2.5v at rest. I took out exactly what I put in, driving pretty hard, having a good time with the top down on a perfectly sunny day in Austin. The Zilla does a good job of cutting current at the end of a bottom balanced pack, so I think there was little if any risk of damaging a blade from overdischarging. I can also better trust the low battery indicator from the Zilla as another last ditch warning for when I need to stop and recharge. I like that. I set that indicator at 172v or avg 2.7v/cell, and the low voltage cutoff at 129v or avg 2.0v/cell.
I didn't bottom balance again. I just charged up and this time I trickle charged at the end and got 74Ah into the pack. That would be about 44 miles range if driven very conservatively. Blade 42 was again the capacity limiting blade, and I stopped the charger at 3.66v. I get the feeling with bottom balancing, that I will be able to put more Ah into the weakest cells. I doubt I will get 96Ah, but I may get 85Ah which would be nice. K2 did tell us to possibly expect some decline in capacity over time, maybe 3-4%/yr. So at 4%, that would be 88Ah. So 85Ah is pretty close and I should be happy with that I guess.
I will drive it to and from work this week, and next weekend I will recheck to see if there has been any drift and re-bottom balance. We should be able to prove whether there is any drift, and if so how often one must re-bottom balance to maintain a safe pack.
As with most things in life, I often have to learn things the hard way. The last deep discharge probably did damage the weakest blade, 42. I don't seem to be getting as much capacity out of the top balanced pack, and I am at risk of running the weakest blades down to reversal every time I drive the car too far. So after some soul searching and a little help from my friends Jack and Tim, I decided to buck from my team at Revolt, and bottom balance. Thankfully, bottom balancing with a BMS was not so painful to accomplish. I drove the car to help discharge the batteries, then I reset the BMS shunts at 2.75v. It took another 36 hours for the top balanced cells to shunt down, and then I gradually charged up the lower cells to 2.75 +/- 0.02v. I then reset the shunts to 3.9v and charged up the pack, manually shutting off the charger when 42-1 got to 3.7v taking in about 71.3Ah.
I drove the car and it was much more predictable discharging and none of the cells went much below 2.5v at rest. I took out exactly what I put in, driving pretty hard, having a good time with the top down on a perfectly sunny day in Austin. The Zilla does a good job of cutting current at the end of a bottom balanced pack, so I think there was little if any risk of damaging a blade from overdischarging. I can also better trust the low battery indicator from the Zilla as another last ditch warning for when I need to stop and recharge. I like that. I set that indicator at 172v or avg 2.7v/cell, and the low voltage cutoff at 129v or avg 2.0v/cell.
I didn't bottom balance again. I just charged up and this time I trickle charged at the end and got 74Ah into the pack. That would be about 44 miles range if driven very conservatively. Blade 42 was again the capacity limiting blade, and I stopped the charger at 3.66v. I get the feeling with bottom balancing, that I will be able to put more Ah into the weakest cells. I doubt I will get 96Ah, but I may get 85Ah which would be nice. K2 did tell us to possibly expect some decline in capacity over time, maybe 3-4%/yr. So at 4%, that would be 88Ah. So 85Ah is pretty close and I should be happy with that I guess.
I will drive it to and from work this week, and next weekend I will recheck to see if there has been any drift and re-bottom balance. We should be able to prove whether there is any drift, and if so how often one must re-bottom balance to maintain a safe pack.
Sunday, February 13, 2011
Overdischarged, but seems to be OK!
Quest for an Electric Porsche Boxster(click for evalbum).
Started with 374Ah on eVision, and drove aggressively today. Did not bring laptop assuming I had a safe 72Ah. I was wrong. When driving hard, more capacity may be lost as heat on high amp loads, but Evision does not account for this and I was not watching pack voltage. When I hit 307Ah (67Ah used) I started to see the low voltage battery indicator from the Zilla at moderate loads, followed by loss of power from the Zilla low voltage cut off, so I pulled off the freeway and into a parking garage as quickly as possible. I immediately turned off the breakers for all the strings to stop any 12V draw. I left the Porsche there for about 20-30 minutes and got a ride back to the house to get a long extension cord to use at a nearby outlet in the parking garage. Here is the BMS readout when I returned with the laptop:
rob@ubuntu:~$ ./summary.sh
spawn [open ...]
blade/millivolt/temp in C (4 columns of cells in a blade)
Rear Pack String:
01 3193 034 3213 034 3198 034 3198 034
02 3200 035 3208 035 3185 035 3192 035
03 2589 037 2712 037 2576 037 2720 037
04 2526 044 2645 044 2574 044 2619 044
05 2516 046 2460 046 2463 046 2505 046
06 3189 043 3197 043 3194 043 3190 043
07 2701 042 2740 042 2751 042 2753 042
08 2867 040 2770 040 2850 040 2844 040
09 2682 039 2718 039 2692 039 2691 039
10 2572 050 2554 050 2706 050 2736 050
11 2533 048 2551 048 2501 048 2513 048
12 2540 049 2559 049 2505 049 2615 049
13 2605 049 2535 049 2578 049 2587 049
14 2517 048 2512 048 2511 048 2597 048
15 2598 046 2570 046 2557 046 2562 046
16 2677 044 2724 044 2722 044 2678 044
Middle Pack String:
17 2773 037 2825 037 2799 037 2780 037
18 2498 046 2529 046 2507 046 2540 046
19 3188 042 3205 042 3185 042 3194 042
20 2463 044 2517 044 2502 044 2487 044
21 3191 042 3212 042 3195 042 3191 042
22 3189 041 3207 041 3194 041 3190 041
23 2633 045 2666 045 2706 045 2720 045
24 3191 036 3208 036 3189 036 3189 036
25 2486 042 2495 042 2575 042 2549 042
26 2587 047 2515 047 2722 047 2692 047
27 2437 053 2462 053 2448 053 2451 053
28 2515 054 2487 054 2475 054 2538 054
29 2728 050 2644 050 2636 050 2681 050
30 2561 047 2509 047 2593 047 2574 047
31 2535 047 2527 047 2460 047 2524 047
32 2495 038 2561 038 2613 038 2636 038
Front Pack String:
33 2573 039 2657 039 2726 039 2777 039
34 2578 046 2562 046 2559 046 2617 046
35 2588 048 2636 048 2541 048 2612 048
36 2780 051 2707 051 2745 051 2809 051
37 2584 048 2530 048 2558 048 2659 048
38 3192 048 3207 048 3198 048 3198 048
39 2708 048 2710 048 2598 048 2689 048
40 3192 049 3211 049 3187 049 3184 049
41 2512 049 2554 049 2518 049 2546 049
42 0723 054 2505 054 2536 054 2543 054
43 2652 051 2638 051 2660 051 2670 051
44 2612 049 2534 049 2570 049 2572 049
45 3186 046 3211 046 3186 046 3188 046
46 2582 045 2568 045 2551 045 2585 045
47 2814 044 2767 044 2784 044 2802 044
48 2612 038 2582 038 2585 038 2624 038
Max Voltage: 3213 (12)
Min Voltage: 723 {421)
Max Temp: 54
Min Temp: 34
After charging 45min at 120V 10A, I was able to drive home safely. I turned off the front pack breaker since 42-1 was at about 2.9V at rest. I charged up the pack and it took in about 85Ah. 42-1 did not charge up first, or lag behind its peers in the same blade, so I think this is a bad blade even before this run. I performed a partial range test on Sunday, and the pack seems fine so far. I will try and perform more range tests after the pack is balanced fully.
Started with 374Ah on eVision, and drove aggressively today. Did not bring laptop assuming I had a safe 72Ah. I was wrong. When driving hard, more capacity may be lost as heat on high amp loads, but Evision does not account for this and I was not watching pack voltage. When I hit 307Ah (67Ah used) I started to see the low voltage battery indicator from the Zilla at moderate loads, followed by loss of power from the Zilla low voltage cut off, so I pulled off the freeway and into a parking garage as quickly as possible. I immediately turned off the breakers for all the strings to stop any 12V draw. I left the Porsche there for about 20-30 minutes and got a ride back to the house to get a long extension cord to use at a nearby outlet in the parking garage. Here is the BMS readout when I returned with the laptop:
rob@ubuntu:~$ ./summary.sh
spawn [open ...]
blade/millivolt/temp in C (4 columns of cells in a blade)
Rear Pack String:
01 3193 034 3213 034 3198 034 3198 034
02 3200 035 3208 035 3185 035 3192 035
03 2589 037 2712 037 2576 037 2720 037
04 2526 044 2645 044 2574 044 2619 044
05 2516 046 2460 046 2463 046 2505 046
06 3189 043 3197 043 3194 043 3190 043
07 2701 042 2740 042 2751 042 2753 042
08 2867 040 2770 040 2850 040 2844 040
09 2682 039 2718 039 2692 039 2691 039
10 2572 050 2554 050 2706 050 2736 050
11 2533 048 2551 048 2501 048 2513 048
12 2540 049 2559 049 2505 049 2615 049
13 2605 049 2535 049 2578 049 2587 049
14 2517 048 2512 048 2511 048 2597 048
15 2598 046 2570 046 2557 046 2562 046
16 2677 044 2724 044 2722 044 2678 044
Middle Pack String:
17 2773 037 2825 037 2799 037 2780 037
18 2498 046 2529 046 2507 046 2540 046
19 3188 042 3205 042 3185 042 3194 042
20 2463 044 2517 044 2502 044 2487 044
21 3191 042 3212 042 3195 042 3191 042
22 3189 041 3207 041 3194 041 3190 041
23 2633 045 2666 045 2706 045 2720 045
24 3191 036 3208 036 3189 036 3189 036
25 2486 042 2495 042 2575 042 2549 042
26 2587 047 2515 047 2722 047 2692 047
27 2437 053 2462 053 2448 053 2451 053
28 2515 054 2487 054 2475 054 2538 054
29 2728 050 2644 050 2636 050 2681 050
30 2561 047 2509 047 2593 047 2574 047
31 2535 047 2527 047 2460 047 2524 047
32 2495 038 2561 038 2613 038 2636 038
Front Pack String:
33 2573 039 2657 039 2726 039 2777 039
34 2578 046 2562 046 2559 046 2617 046
35 2588 048 2636 048 2541 048 2612 048
36 2780 051 2707 051 2745 051 2809 051
37 2584 048 2530 048 2558 048 2659 048
38 3192 048 3207 048 3198 048 3198 048
39 2708 048 2710 048 2598 048 2689 048
40 3192 049 3211 049 3187 049 3184 049
41 2512 049 2554 049 2518 049 2546 049
42 0723 054 2505 054 2536 054 2543 054
43 2652 051 2638 051 2660 051 2670 051
44 2612 049 2534 049 2570 049 2572 049
45 3186 046 3211 046 3186 046 3188 046
46 2582 045 2568 045 2551 045 2585 045
47 2814 044 2767 044 2784 044 2802 044
48 2612 038 2582 038 2585 038 2624 038
Max Voltage: 3213 (12)
Min Voltage: 723 {421)
Max Temp: 54
Min Temp: 34
After charging 45min at 120V 10A, I was able to drive home safely. I turned off the front pack breaker since 42-1 was at about 2.9V at rest. I charged up the pack and it took in about 85Ah. 42-1 did not charge up first, or lag behind its peers in the same blade, so I think this is a bad blade even before this run. I performed a partial range test on Sunday, and the pack seems fine so far. I will try and perform more range tests after the pack is balanced fully.
Sunday, February 6, 2011
PERFORMANCE
Quest for an Electric Porsche Boxster.
The car is really fun now. Except for range and battery capacity, the car is meeting expectations as far as performance. After emailing Tim Catellier who has a lot of experience with his BMW Z3, and discussing the batteries with David Anderson at K2, I decided to reduce the low voltage limit on the Zilla to an average of 2.3V. This really loosened up the amp output, and now I am getting close to 840A on demand, and more rapid output overall. That means more acceleration! I roughly measured 0-60 times using my cell phone's stopwatch, and on relatively flat ground I am seeing less than 7 seconds, maybe 6, in optimal conditions. That means a full pack and relatively warm weather. The battery voltages sag on heavy load, with the lowest blades at 2.3-2.4V, and an average minimum of around 150V. So one must wonder what the car would be like with a higher nominal voltage pack, say 260V? This would help cold weather performance too.
Austin, Texas has been rather cold in recent days. Last Wed, the temp dropped below 25 degrees. I left home with a half full pack and kept the car garaged that night. So the pack was relatively warm. But at the end of the day, the car was in less than 23 degree weather parked outside, not plugged in. The car was sluggish, and getting on the freeway was barely possible. The car requires about 100-130A to maintain speed on the freeway on level road or with slight elevations. The pack would not produce more than about 160A on the way home limiting acceleration severely. Granted the pack was more than half empty, but compared to warm weather this was a dramatic loss of power. Despite average voltages sagging as low as 2.3v, I was able to get home and the voltages returned to over 3.1v at rest. I tested range in warm weather the next day, and capacity was back at 72Ah. So it was evident the batteries can tolerate battery sag, at least above 2V briefly, without serious problems. So yesterday I decided to drop the Zilla limit to an average minimum of 2.3V. Now in warm weather the car feels like a PORSCHE. Much better than stock! I imagine driving like that may limit range even more, but it's worth it!
The car is really fun now. Except for range and battery capacity, the car is meeting expectations as far as performance. After emailing Tim Catellier who has a lot of experience with his BMW Z3, and discussing the batteries with David Anderson at K2, I decided to reduce the low voltage limit on the Zilla to an average of 2.3V. This really loosened up the amp output, and now I am getting close to 840A on demand, and more rapid output overall. That means more acceleration! I roughly measured 0-60 times using my cell phone's stopwatch, and on relatively flat ground I am seeing less than 7 seconds, maybe 6, in optimal conditions. That means a full pack and relatively warm weather. The battery voltages sag on heavy load, with the lowest blades at 2.3-2.4V, and an average minimum of around 150V. So one must wonder what the car would be like with a higher nominal voltage pack, say 260V? This would help cold weather performance too.
Austin, Texas has been rather cold in recent days. Last Wed, the temp dropped below 25 degrees. I left home with a half full pack and kept the car garaged that night. So the pack was relatively warm. But at the end of the day, the car was in less than 23 degree weather parked outside, not plugged in. The car was sluggish, and getting on the freeway was barely possible. The car requires about 100-130A to maintain speed on the freeway on level road or with slight elevations. The pack would not produce more than about 160A on the way home limiting acceleration severely. Granted the pack was more than half empty, but compared to warm weather this was a dramatic loss of power. Despite average voltages sagging as low as 2.3v, I was able to get home and the voltages returned to over 3.1v at rest. I tested range in warm weather the next day, and capacity was back at 72Ah. So it was evident the batteries can tolerate battery sag, at least above 2V briefly, without serious problems. So yesterday I decided to drop the Zilla limit to an average minimum of 2.3V. Now in warm weather the car feels like a PORSCHE. Much better than stock! I imagine driving like that may limit range even more, but it's worth it!
Friday, January 28, 2011
Back on the ROAD!
Quest for an Electric Porsche Boxster.
Finally back on the road today after a lot of work. Netgain diagnosed a fan failure and sent us a new fan. We tried to follow instructions, but the fan is so tightly fixed to the shaft, we broke the replacement fan trying to install it incorrectly. We paid for another fan, about $240, and used a PVC pipe over the motor shaft, with a 2x4 on top of the pipe, and a hammer on the 2x4 to force the fan onto the shaft. It took an hour of hammering, checking, hammering, etc. Finally got the fan on, reassembled and presto... IT SPUN UP! So, almost 4 weeks later and about 8 hours of work tearing down, fixing motor and reassembly. Back on the road!
If this happens to you, strongly consider sending back to Netgain for repairs, as if you break the fan you will have to pay for another and it will delay the repair by at least a week. If you repair it yourself, DO NOT HAMMER ON THE OUTER RIM OF THE FAN. It will not work and the fan is very strong, but brittle like ceramic. IT WILL CRACK. Also be sure and remove as many fragments as possible using a powerful vacuum and forced air if available. We had a small fragment we missed and were worried it caused some damage. Had to take the fan grate off and remove it. The fragment fell out, and it was fine afterwards with no damage to the new fan.
Finally back on the road today after a lot of work. Netgain diagnosed a fan failure and sent us a new fan. We tried to follow instructions, but the fan is so tightly fixed to the shaft, we broke the replacement fan trying to install it incorrectly. We paid for another fan, about $240, and used a PVC pipe over the motor shaft, with a 2x4 on top of the pipe, and a hammer on the 2x4 to force the fan onto the shaft. It took an hour of hammering, checking, hammering, etc. Finally got the fan on, reassembled and presto... IT SPUN UP! So, almost 4 weeks later and about 8 hours of work tearing down, fixing motor and reassembly. Back on the road!
If this happens to you, strongly consider sending back to Netgain for repairs, as if you break the fan you will have to pay for another and it will delay the repair by at least a week. If you repair it yourself, DO NOT HAMMER ON THE OUTER RIM OF THE FAN. It will not work and the fan is very strong, but brittle like ceramic. IT WILL CRACK. Also be sure and remove as many fragments as possible using a powerful vacuum and forced air if available. We had a small fragment we missed and were worried it caused some damage. Had to take the fan grate off and remove it. The fragment fell out, and it was fine afterwards with no damage to the new fan.
Friday, January 7, 2011
Warp 11 Fan Destroyed, Netgain to Rescue!
Quest for an Electric Porsche Boxster.
Here are some photos of the fan failure... Netgain is sending a replacement and we will hopefully have it fixed in a week or so without having to ship it back.
Here are some photos of the fan failure... Netgain is sending a replacement and we will hopefully have it fixed in a week or so without having to ship it back.
Saturday, January 1, 2011
Warp 11 Motor failure
Quest for an Electric Porsche Boxster.
Drove out to the lake and back. Something came loose in the motor when driving in 2nd gear up to about 4000rpm when leaving the marina parking lot . It clinked and scratched loudly at first, as if bouncing around, and eventually settled down, temporarily went away at lower rpms. I was able to drive home and heard the noises a few more times when going up hills or perhaps bumps as if something plastic was bouncing around in there. It wasn't as loud as at first. It would settle down on even road and had no perceptible noise for fairly long distances at 2500rpm going about 50-55mph. Once home, I tested the motor in neutral and you can hear scraping sounds. I think this motor is going to need TLC from Netgain.
On a lighter note, replacing the motor mount has made a noticable difference on take offs, and overall the handling going to the lake was very good. The efficiency on the trip was about 1.8Ah/mile which is pretty good, and power performance is pretty good too, particularly since lowering the minimum voltage limit on the Zilla. I don't think there is much more room for maximizing performance with this battery pack however, and I may swap out the Zilla Z2K for a Z1K as that should be more than enough for this pack.
Drove out to the lake and back. Something came loose in the motor when driving in 2nd gear up to about 4000rpm when leaving the marina parking lot . It clinked and scratched loudly at first, as if bouncing around, and eventually settled down, temporarily went away at lower rpms. I was able to drive home and heard the noises a few more times when going up hills or perhaps bumps as if something plastic was bouncing around in there. It wasn't as loud as at first. It would settle down on even road and had no perceptible noise for fairly long distances at 2500rpm going about 50-55mph. Once home, I tested the motor in neutral and you can hear scraping sounds. I think this motor is going to need TLC from Netgain.
On a lighter note, replacing the motor mount has made a noticable difference on take offs, and overall the handling going to the lake was very good. The efficiency on the trip was about 1.8Ah/mile which is pretty good, and power performance is pretty good too, particularly since lowering the minimum voltage limit on the Zilla. I don't think there is much more room for maximizing performance with this battery pack however, and I may swap out the Zilla Z2K for a Z1K as that should be more than enough for this pack.
Battery Capacity and Range part II
Quest for an Electric Porsche Boxster.
I did another capacity test yesterday. Started at 276Ah and went to 204Ah limping into the garage for a total of 72Ah. Blade 33 performed well, but this time 35-2 went south in hurry. I think the shoulder on this pack is about 2.75V with light amp draw of about 60-100A. At brief rest, 35-2 went back to 2.9V so I decided to go around the block again, which ended up being about 2.6 miles. Too much, and I briefly went to 1.4V on 35-2 when drawing 50-60A getting back to the house. I wish I could have taken 1 mile back. It went back up to 1.9V after brief rest. Most other cells including blade 33 went over 3V and a few were at 2.9V. The other cells in 35 were not as weak as 35-2. Anyway, I don't think blade 33 is a problem anymore than the general capacity of the pack. Various blades took turns as the minimum voltage with accelerations during the drive, and cells in blade 33 only came up once I think. Blade 35 came up a few times, but I wouldn't have singled it out earlier in the drive. So right now I think we have a fairly stable pack, of around 70Ah (safely at 1.8Ah/mile).
I left the charger on 2A overnight which eVision showed about 0.5A going to the pack. Before going to bed the pack was at 278Ah with 42-1 reaching shunt at around 3.56V, and the rest of the pack still around 3.3-3.4V. The pack balanced about 7.5 hours overnight to the following...
B# mV T
cell 1 cell 2 cell 3 cell 4
01 3575 024 3583 024 3573 024 3584 024
02 3573 025 3577 025 3575 025 3576 025
03 3571 020 3576 020 3578 020 3578 020
04 3575 028 3577 028 3578 028 3577 028
05 3578 028 3578 028 3576 028 3575 028
06 3578 028 3576 028 3571 028 3571 028
07 3577 028 3577 028 3576 028 3572 028
08 3580 028 3577 028 3579 028 3574 028
09 3580 026 3579 026 3579 026 3579 026
10 3580 032 3579 032 3571 032 3570 032
11 3577 029 3576 029 3573 029 3575 029
12 3576 031 3579 031 3575 031 3573 031
13 3579 031 3579 031 3576 031 3577 031
14 3574 032 3576 032 3574 032 3575 032
15 3577 029 3577 029 3575 029 3574 029
16 3582 031 3583 031 3583 031 3582 031
17 3565 028 3570 028 3572 028 3572 028
18 3577 033 3577 033 3571 033 3572 033
19 3576 028 3577 028 3574 028 3574 028
20 3575 033 3575 033 3572 033 3576 033
21 3572 029 3572 029 3566 029 3566 029
22 3572 029 3573 029 3570 029 3571 029
23 3576 033 3577 033 3571 033 3571 033
24 3574 026 3575 026 3571 026 3569 026
25 3576 028 3576 028 3572 028 3571 028
26 3571 029 3573 029 3569 029 3566 029
27 3573 032 3572 032 3566 032 3564 032
28 3574 033 3575 033 3570 033 3572 033
29 3571 031 3571 031 3566 031 3565 031
30 3574 027 3576 027 3572 027 3572 027
31 3570 029 3573 029 3570 029 3568 029
32 3556 026 3561 026 3570 026 3567 026
33 3573 029 3571 029 3570 029 3571 029
34 3567 032 3567 032 3568 032 3565 032
35 3573 033 3575 033 3570 033 3570 033
36 3572 035 3574 035 3569 035 3572 035
37 3573 034 3573 034 3574 034 3574 034
38 3576 036 3575 036 3571 036 3571 036
39 3572 035 3572 035 3567 035 3566 035
40 3570 037 3567 037 3570 037 3568 037
41 3570 036 3571 036 3566 036 3564 036
42 3569 035 3571 035 3569 035 3567 035
43 3569 035 3572 035 3569 035 3566 035
44 3572 035 3572 035 3567 035 3566 035
45 3567 034 3566 034 3567 034 3566 034
46 3574 032 3573 032 3572 032 3572 032
47 3573 032 3574 032 3564 032 3565 032
48 3570 028 3571 028 3570 028 3568 028
Max Voltage: 3584 (14)
Min Voltage: 3556 {321
Max Temp: 37
Min Temp: 20
Most all the cells showed the orange lights shunting, so not all the Ahrs overnight shown on eVision went in the pack. Nonetheless the total on eVision was 285Ahrs from 204Ahrs before charging. So, even including shunt losses, the pack only took in about 81Ahrs. 35-2 was not the first to reach full, and so I don't think we have anymore blades that stand out as being bad. We just have a lower than expected usable capacity for this pack. If it is supposed to be 96Ahrs, then we are still missing at least 16-20% capacity. Unless something else is wrong, I think we have no more than 80Ahrs or about 47miles range at 1.7Ah/m at the very maximum.
I don't see this pack safely producing 10C continuous discharge rates. It barely produces 5-6C with a full pack. 5C discharge is what K2 lists as "nominal capacity" on their spec sheet. Without lowering the Zilla voltage limit further, 5-6C is probably the maximum useful draw on demand for acceleration in our arrangement. Therefore, the Z2K is probably not going to be better than the Z1K with this pack producing around 500-600A maximum. If I recalculate the cell capacity from the expected 3200mAh down to 2700mAh (about what we are seeing now), then discharge rates appear higher. So with 500A on acceleration, that results in a 6C discharge rate based on the lower capacity. 630A, which I saw once with a full pack, is about 7.7C, which is not too bad and overall performance is pretty good but not as exciting as we hoped.
Digesting all this, I guess we would like to know the maximum tolerable voltage limit we can set on the Zilla for brief accelerations without "abusing" the cells, since we are not seeing much beyond 5-6C discharge rates with Zilla set at average 2.4V sag. Will the cells tolerate going to 2.0V sag for less than 30sec? How about 1.5V, if brief rest brings them back to over 3V? And finally, why are we not getting closer to expected capacity from this configuration? These are all questions I will need to review with K2 to see if we are missing something or if they have any suggestions. These were much more expensive batteries than the Thundersky variety, and I am not seeing the energy density or power advantage at this point.
I did another capacity test yesterday. Started at 276Ah and went to 204Ah limping into the garage for a total of 72Ah. Blade 33 performed well, but this time 35-2 went south in hurry. I think the shoulder on this pack is about 2.75V with light amp draw of about 60-100A. At brief rest, 35-2 went back to 2.9V so I decided to go around the block again, which ended up being about 2.6 miles. Too much, and I briefly went to 1.4V on 35-2 when drawing 50-60A getting back to the house. I wish I could have taken 1 mile back. It went back up to 1.9V after brief rest. Most other cells including blade 33 went over 3V and a few were at 2.9V. The other cells in 35 were not as weak as 35-2. Anyway, I don't think blade 33 is a problem anymore than the general capacity of the pack. Various blades took turns as the minimum voltage with accelerations during the drive, and cells in blade 33 only came up once I think. Blade 35 came up a few times, but I wouldn't have singled it out earlier in the drive. So right now I think we have a fairly stable pack, of around 70Ah (safely at 1.8Ah/mile).
I left the charger on 2A overnight which eVision showed about 0.5A going to the pack. Before going to bed the pack was at 278Ah with 42-1 reaching shunt at around 3.56V, and the rest of the pack still around 3.3-3.4V. The pack balanced about 7.5 hours overnight to the following...
B# mV T
cell 1 cell 2 cell 3 cell 4
01 3575 024 3583 024 3573 024 3584 024
02 3573 025 3577 025 3575 025 3576 025
03 3571 020 3576 020 3578 020 3578 020
04 3575 028 3577 028 3578 028 3577 028
05 3578 028 3578 028 3576 028 3575 028
06 3578 028 3576 028 3571 028 3571 028
07 3577 028 3577 028 3576 028 3572 028
08 3580 028 3577 028 3579 028 3574 028
09 3580 026 3579 026 3579 026 3579 026
10 3580 032 3579 032 3571 032 3570 032
11 3577 029 3576 029 3573 029 3575 029
12 3576 031 3579 031 3575 031 3573 031
13 3579 031 3579 031 3576 031 3577 031
14 3574 032 3576 032 3574 032 3575 032
15 3577 029 3577 029 3575 029 3574 029
16 3582 031 3583 031 3583 031 3582 031
17 3565 028 3570 028 3572 028 3572 028
18 3577 033 3577 033 3571 033 3572 033
19 3576 028 3577 028 3574 028 3574 028
20 3575 033 3575 033 3572 033 3576 033
21 3572 029 3572 029 3566 029 3566 029
22 3572 029 3573 029 3570 029 3571 029
23 3576 033 3577 033 3571 033 3571 033
24 3574 026 3575 026 3571 026 3569 026
25 3576 028 3576 028 3572 028 3571 028
26 3571 029 3573 029 3569 029 3566 029
27 3573 032 3572 032 3566 032 3564 032
28 3574 033 3575 033 3570 033 3572 033
29 3571 031 3571 031 3566 031 3565 031
30 3574 027 3576 027 3572 027 3572 027
31 3570 029 3573 029 3570 029 3568 029
32 3556 026 3561 026 3570 026 3567 026
33 3573 029 3571 029 3570 029 3571 029
34 3567 032 3567 032 3568 032 3565 032
35 3573 033 3575 033 3570 033 3570 033
36 3572 035 3574 035 3569 035 3572 035
37 3573 034 3573 034 3574 034 3574 034
38 3576 036 3575 036 3571 036 3571 036
39 3572 035 3572 035 3567 035 3566 035
40 3570 037 3567 037 3570 037 3568 037
41 3570 036 3571 036 3566 036 3564 036
42 3569 035 3571 035 3569 035 3567 035
43 3569 035 3572 035 3569 035 3566 035
44 3572 035 3572 035 3567 035 3566 035
45 3567 034 3566 034 3567 034 3566 034
46 3574 032 3573 032 3572 032 3572 032
47 3573 032 3574 032 3564 032 3565 032
48 3570 028 3571 028 3570 028 3568 028
Max Voltage: 3584 (14)
Min Voltage: 3556 {321
Max Temp: 37
Min Temp: 20
Most all the cells showed the orange lights shunting, so not all the Ahrs overnight shown on eVision went in the pack. Nonetheless the total on eVision was 285Ahrs from 204Ahrs before charging. So, even including shunt losses, the pack only took in about 81Ahrs. 35-2 was not the first to reach full, and so I don't think we have anymore blades that stand out as being bad. We just have a lower than expected usable capacity for this pack. If it is supposed to be 96Ahrs, then we are still missing at least 16-20% capacity. Unless something else is wrong, I think we have no more than 80Ahrs or about 47miles range at 1.7Ah/m at the very maximum.
I don't see this pack safely producing 10C continuous discharge rates. It barely produces 5-6C with a full pack. 5C discharge is what K2 lists as "nominal capacity" on their spec sheet. Without lowering the Zilla voltage limit further, 5-6C is probably the maximum useful draw on demand for acceleration in our arrangement. Therefore, the Z2K is probably not going to be better than the Z1K with this pack producing around 500-600A maximum. If I recalculate the cell capacity from the expected 3200mAh down to 2700mAh (about what we are seeing now), then discharge rates appear higher. So with 500A on acceleration, that results in a 6C discharge rate based on the lower capacity. 630A, which I saw once with a full pack, is about 7.7C, which is not too bad and overall performance is pretty good but not as exciting as we hoped.
Digesting all this, I guess we would like to know the maximum tolerable voltage limit we can set on the Zilla for brief accelerations without "abusing" the cells, since we are not seeing much beyond 5-6C discharge rates with Zilla set at average 2.4V sag. Will the cells tolerate going to 2.0V sag for less than 30sec? How about 1.5V, if brief rest brings them back to over 3V? And finally, why are we not getting closer to expected capacity from this configuration? These are all questions I will need to review with K2 to see if we are missing something or if they have any suggestions. These were much more expensive batteries than the Thundersky variety, and I am not seeing the energy density or power advantage at this point.
Saturday, December 11, 2010
Battery Capacity and Range
Quest for an Electric Porsche Boxster.
I went to the Chevy dealer in Austin yesterday, lucky to be in one of the early release markets. The sales lady said all the Volts are accounted for thru April or June of 2011, and they are selling them at $55K! She said there is a waiting list 39 deep, which I suppose means beyond the ones already alloted to the dealership. So I won't hold my breath waiting for a Volt.
As for battery management on the Porsche, we are top balancing K2 Energy, 26650EV blades. I am finding lower than expected range from the pack so far, presumably due to weak or shorted cells. There are 1920 cells arranged in spot welded arrays: 10 cells in parallel columns, with 4 rows in series per blade. We have column level battery management shunting up to 0.5A while top balancing.
Top balancing takes a long time and I don't think we have completely top balanced yet, with about 0.1-0.2V difference between cells at the top of charging. So far, I haven't burned down the house, and my wife and I (and two chihuahuas) sleep right above our gargage. I don't charge at night while asleep after following Jack's EVTV blog. http://web.me.com/mjrickard/
We are still getting acquainted with measuring pack capacity. We carefully monitor the voltage toward the end of pack capacity using a laptop, and still don't have the eVision set up properly. It seems using eVision for measuring Ah consumed while driving works well. We are seeing about 320 whr/mile with conservative driving, mostly freeway, at 60-65mph. So we should get close to 60 miles range with a 20KWH pack, similar to Tim's BMW Z3. http://www.evalbum.com/3189 However I think we are about 10 miles and 20 Ahrs shy.
When initially testing max range, there was a column that was consistently about a 0.3-0.4V below the rest of the columns, markedly limiting capacity of the pack. We replaced that blade with a backup, and range improved about 5+ miles. Now, there is another column that is about 0.3V below the rest of the columns toward end of pack capacity, and we are only getting about 75Ah out of what should be a 96Ah pack. The batteries did sit for 2 years while we experienced delays building the car. So I was expecting some loss of capacity, but not this much.
I have driven the car about 250 miles so far, and it is great fun! I intend to use it as my daily commuter car once it is more reliable. Unfortunately, the left rear CV joint suddenly unscrewed itself while driving 2 days ago in heavy traffic, so we are in the shop for awhile waiting for parts. :)
I went to the Chevy dealer in Austin yesterday, lucky to be in one of the early release markets. The sales lady said all the Volts are accounted for thru April or June of 2011, and they are selling them at $55K! She said there is a waiting list 39 deep, which I suppose means beyond the ones already alloted to the dealership. So I won't hold my breath waiting for a Volt.
As for battery management on the Porsche, we are top balancing K2 Energy, 26650EV blades. I am finding lower than expected range from the pack so far, presumably due to weak or shorted cells. There are 1920 cells arranged in spot welded arrays: 10 cells in parallel columns, with 4 rows in series per blade. We have column level battery management shunting up to 0.5A while top balancing.
Top balancing takes a long time and I don't think we have completely top balanced yet, with about 0.1-0.2V difference between cells at the top of charging. So far, I haven't burned down the house, and my wife and I (and two chihuahuas) sleep right above our gargage. I don't charge at night while asleep after following Jack's EVTV blog. http://web.me.com/mjrickard/
We are still getting acquainted with measuring pack capacity. We carefully monitor the voltage toward the end of pack capacity using a laptop, and still don't have the eVision set up properly. It seems using eVision for measuring Ah consumed while driving works well. We are seeing about 320 whr/mile with conservative driving, mostly freeway, at 60-65mph. So we should get close to 60 miles range with a 20KWH pack, similar to Tim's BMW Z3. http://www.evalbum.com/3189 However I think we are about 10 miles and 20 Ahrs shy.
When initially testing max range, there was a column that was consistently about a 0.3-0.4V below the rest of the columns, markedly limiting capacity of the pack. We replaced that blade with a backup, and range improved about 5+ miles. Now, there is another column that is about 0.3V below the rest of the columns toward end of pack capacity, and we are only getting about 75Ah out of what should be a 96Ah pack. The batteries did sit for 2 years while we experienced delays building the car. So I was expecting some loss of capacity, but not this much.
I have driven the car about 250 miles so far, and it is great fun! I intend to use it as my daily commuter car once it is more reliable. Unfortunately, the left rear CV joint suddenly unscrewed itself while driving 2 days ago in heavy traffic, so we are in the shop for awhile waiting for parts. :)
Sunday, October 24, 2010
Three battery strings, starting performance testing
Quest for an Electric Porsche Boxster.
Battery input limited to 450A for safety and break in. Controller output voltage limited to 2000A and 170V. K2 recommended max performance for our pack will be up to 840A input for 30s hopefully without significant damage to batteries. At low speeds we think the controller will convert high voltage at low speed into more amperage making better use of the Zilla Z2K 2000A potential.
Flywheel required machine work to resolve clutch engagement noise scraping adapter plate. Realignment cured the loose rear end. Drove it about 12 miles today without much change in battery voltage. It is fast. More performance testing in next 2 weeks.
Battery input limited to 450A for safety and break in. Controller output voltage limited to 2000A and 170V. K2 recommended max performance for our pack will be up to 840A input for 30s hopefully without significant damage to batteries. At low speeds we think the controller will convert high voltage at low speed into more amperage making better use of the Zilla Z2K 2000A potential.
Flywheel required machine work to resolve clutch engagement noise scraping adapter plate. Realignment cured the loose rear end. Drove it about 12 miles today without much change in battery voltage. It is fast. More performance testing in next 2 weeks.
Sunday, September 5, 2010
Front and Mid Battery Strings Installed (still missing rear string)
Quest for an Electric Porsche Boxster.
Now with 2 battery strings installed. Other than road noise, the car is pretty quiet. There is a mild brushing sound from the motor as it spins up, and a catchy noise when activating the clutch. The rear of the car seems a bit unstable making turns under power, which we think is related to alignment. The weight of the car in the rear is lighter than stock, especially without the third battery string installed. The car is torquey with gentle driving. So far, I am impressed that the car works, and pretty well at this stage!
Now with 2 battery strings installed. Other than road noise, the car is pretty quiet. There is a mild brushing sound from the motor as it spins up, and a catchy noise when activating the clutch. The rear of the car seems a bit unstable making turns under power, which we think is related to alignment. The weight of the car in the rear is lighter than stock, especially without the third battery string installed. The car is torquey with gentle driving. So far, I am impressed that the car works, and pretty well at this stage!
Sunday, August 8, 2010
Rebirth of the Porsche Boxster into Nothing's Shocking!
Quest for an Electric Porsche Boxster.
Today, Sunday August 8th, 2010 is the first birthday of Nothing's Shocking, a lithium electric conversion of a 1999 Porsche Boxster. Although not completely finished, Mark and Aaron have achieved sufficient progress to mark this day a major success. The car was test driven a short distance before the steering linkage disengaged. The car drove with only one battery string out of three feeding the Zilla Z2K,with amperage limited to 200A to protect the batteries. The performance was better than expected with surprising torque in 3rd gear. We struggled to get the car back into the garage because of the steering problem. Once inside on the racks, the steering problem was related to incorrect reassembly after removal of the gas tank. We are expecting rapid completion of the other battery strings in the next couple of weeks, and are hopeful the car will exceed predicted specs!
Tuesday, July 27, 2010
Progress and Delays
Quest for an Electric Porsche Boxster.
Select email exchanges between members of REVOLT Custom Electric during the conversion process:
7/2009
I didn't cover the topic of clutch grip as thoroughly as I should have while you were here. There are two things that affect it (primarily) -- material choice and spring pressure. Not talking about the springs in the hub of the clutch disk anymore, this time I'm referring to the springs in the pressure plate assembly itself, that force the ring-shaped plate onto the side of the clutch disk when you release the clutch pedal.
With the higher torque from the Warp11 at 2000 amps, there is a chance that the stock clutch will slip. If it does, then this is a situation that will cause the clutch disk to more quickly degrade over time, but the rate of additional wear would be uncertain and whether this would be a problem at all depends on what kind of torque the stock assembly is already designed to handle. A racing pressure plate, in addition to being lighter, will have stiffer springs to provide more clutch grip. In addition, aftermarket clutch disks are available that provide different materials like ceramic or carbon based pads, to provide more grip than the stock part. Mark went with one of these ceramic disks in his MR2, and it solved his problems with clutch slip (with a 1000A controller and a 10.7" Kostov motor that's shorter than yours).
Unless we can find more data about the stock parts and at what torque they're known to begin slipping, we're really sort of in the dark about the decision. The racing parts may give a definite benefit in reduced weight, but beyond that you'd essentially be purchasing "insurance" against having to disassemble the powertrain again to install improved parts later. I've been thinking that the Porsche parts might be enough and generally aiming to err on the side of keeping the project costs down; perhaps this is unwise as a policy.
Chris
--------------------------------------------------------------------------
2/2010
Aaron
--------------------------------------------------------------------------
At this point, the boxes have been built with an inch of air gap above and below the cells and we will be seperating the cells by about a 1/16 of an inch. We're told that this should be sufficient on its own to prevent them from overheating and our experience with the Mazda project seems to show that the cells shouldn't heat up much under load. So, for now, there won't be any penetrations in the boxes. If it proves to be an issue, we can add holes to allow more airflow through the boxes. If that isn't sufficient, we can add fans to do forced air cooling. That's not worth the expense if they aren't necessary.
I realized I got side tracked during our conversation and we never completed discussing the DC/DC. Our preference would be to use the Iota 55amp 220V Converter as a DC/DC. I have a new one handy, and
the size is about right to fit near the 12v battery up front. If you want to explore other options just let us know. I don't have much data on using the Vicor modules, so any pointers would be helpful.
Mark
The AC normally is pretty easy with a "close clutch"signal from the climate system. Right now that signal never appears to be triggered. My guess is the climate computer won't try to turn on the AC clutch unless the engine is idling, maybe? Seems odd, the signal doesn't go from the climate system straight to the clutch, it
passed thru the Engine computer which decides when to close the clutch. One would think the ECU could disable the clutch if the engine wasn't running. There is two signals going to the hot/cold mixing valve that we might be able to use instead, but it means disassembling the dash enough to tap that signal... not fun.
Alternative might be to put a tiny three position switch on the dash for the heater/AC or off.
Mark
Finally figured out the AC control circuitry. I've been trying to figure out why the climate control computer never asserted the signal line asking for the AC compressor to be turned on even when set to max
cooling. My suspicion was that it was waiting to see the engine running, and tonight I figured out a way to fake an idle signal to the dash using a speed sensor and my cordless drill.
The good news is that it works, once it sees the engine idling for about 30 seconds it starts pulsing the AC demand line.
Bad news... the pulsing won't turn on the Masterflux compressor, I need some circuitry to convert the pulsing signal to a continuous on signal for the masterflux controller. Other bad news... our motor doesn't idle, so the AC won't turn on until your going down the road and might turn off at traffic lights. So we probably need to build a circuit that sends a false "idling" tach signal to the dash when the motor is stopped. I'm going to ask Otmar if he might be able to add this to the Zilla software, it would be really easy for him. Otherwise I have to design a circuit and send out to have the PC board made. (Time consuming)
Mark
--------------------------------------------------------------------------
If it is a 205V pack, and we can output a max of 960A, I am wondering what the motor will actually see on early acceleration? Can the controller convert extra voltage not used at lower motor rpms into amps for the motor???
Rob
The large inductance of the motor and the switching of the controller produces what electrical engineers call a "buck" DC/DC converter. A Buck converter can step output voltage down and output current up. At low speeds the motor only needs 80-100V, so the output or motor amps might be double input (battery) amps. But the controller's 1000 or 2000amp rating is on the motor side. (Mostly the freewheel diodes, which work the hardest at slow speeds) As speed increases the motor wants to become a generator, so the motor side voltage has to go higher and higher to force amps into the motor and amps start falling off. Top speed is limited by how much voltage you can use to stuff
So in general, a 1000amp motor controller will never draw 1000amps on the battery side. At the most it will draw about 800, and usually more like 600-700.
A 2000 amp controller could draw up to 1200-1500 battery amps... but there are few batteries out there that can produce that for more than a second or two.
Mark
--------------------------------------------------------------------------
According to the 10C discharge curve on the web site I sent you, a mid-way discharged pack will drop from 3.2V to 2.4V under 10C load which is actually a 25% sag. That would give us 153V at presumably a full 960A. If we are using 100V, then we convert the extra 53V into a relative increase in Amps, which may be an additional 333A. So total, at 100V, we have 1293A to the motor, even more at lower speeds. Based on this concept, at low RPM, could we give the full 2000A capability of the Z2K to the motor with our pack? Acceleration will taper off as motor rpms increase due to higher voltage diminishing amp capability of the controller.
Rob
Its very complex, and difficult to model. The exact curve the batteries give will be a big factor. 20% sag may be optimistic, though these cells might surprise us. Since we have the option to try it first, lets see what the the difference is. We could even put the car on a dyno with each controller and see if there is a difference.
It may also be that a 1k produces the acceleration you want.
Mark
Select email exchanges between members of REVOLT Custom Electric during the conversion process:
7/2009
I didn't cover the topic of clutch grip as thoroughly as I should have while you were here. There are two things that affect it (primarily) -- material choice and spring pressure. Not talking about the springs in the hub of the clutch disk anymore, this time I'm referring to the springs in the pressure plate assembly itself, that force the ring-shaped plate onto the side of the clutch disk when you release the clutch pedal.
With the higher torque from the Warp11 at 2000 amps, there is a chance that the stock clutch will slip. If it does, then this is a situation that will cause the clutch disk to more quickly degrade over time, but the rate of additional wear would be uncertain and whether this would be a problem at all depends on what kind of torque the stock assembly is already designed to handle. A racing pressure plate, in addition to being lighter, will have stiffer springs to provide more clutch grip. In addition, aftermarket clutch disks are available that provide different materials like ceramic or carbon based pads, to provide more grip than the stock part. Mark went with one of these ceramic disks in his MR2, and it solved his problems with clutch slip (with a 1000A controller and a 10.7" Kostov motor that's shorter than yours).
Unless we can find more data about the stock parts and at what torque they're known to begin slipping, we're really sort of in the dark about the decision. The racing parts may give a definite benefit in reduced weight, but beyond that you'd essentially be purchasing "insurance" against having to disassemble the powertrain again to install improved parts later. I've been thinking that the Porsche parts might be enough and generally aiming to err on the side of keeping the project costs down; perhaps this is unwise as a policy.
Chris
--------------------------------------------------------------------------
2/2010
At his point, it looks like we'll be placing one string each in the front trunk, the engine bay (split on either side of the motor), and then also in the rear. This should allow for us to service the blades from above without a great deal of parts removal. I am doing some final modeling to make sure that the rear most box will not extend below the car too much. It may take up a significant chunk of the rear trunk though.
Aaron
--------------------------------------------------------------------------
6/2010
At this point, the boxes have been built with an inch of air gap above and below the cells and we will be seperating the cells by about a 1/16 of an inch. We're told that this should be sufficient on its own to prevent them from overheating and our experience with the Mazda project seems to show that the cells shouldn't heat up much under load. So, for now, there won't be any penetrations in the boxes. If it proves to be an issue, we can add holes to allow more airflow through the boxes. If that isn't sufficient, we can add fans to do forced air cooling. That's not worth the expense if they aren't necessary.
Aaron
--------------------------------------------------------------------------
The boxes look sturdy and well made. They will probably be safe in a collision, which was a concern of mine. They are not exactly on track for the rear box and say they will need to split it into 3 boxes to make them fit. They hope to get that done in the next few days, and I think they will need to get busy to make it so.
Will there be a temperature sensor in the boxes to detect overheating? There is a surprising amount of space still left above the motor and lateral battery boxes. Are the plans to mount the Zilla, Manzanita and or DC/DC converter above the battery boxes and motor? Do you have the DC/DC converter yet? What are plans about power steering?
--------------------------------------------------------------------------
The boxes look sturdy and well made. They will probably be safe in a collision, which was a concern of mine. They are not exactly on track for the rear box and say they will need to split it into 3 boxes to make them fit. They hope to get that done in the next few days, and I think they will need to get busy to make it so.
Will there be a temperature sensor in the boxes to detect overheating? There is a surprising amount of space still left above the motor and lateral battery boxes. Are the plans to mount the Zilla, Manzanita and or DC/DC converter above the battery boxes and motor? Do you have the DC/DC converter yet? What are plans about power steering?
There are two temperature sensors per blade. One is monitoring the board temperature ( to make sure the resistors aren't heating up too much) and another to monitor the temp of the cells that comprise the blade. We'll have plenty of monitoring to detect temp issues.
The battery boxes are set so that we can load them from above. Also, as you suspect, the controller and a fair amount of electronics, wiring, contactors, breakers and such will be installed above them. It is that install that we will be working to complete by end of July.
We do have the dc/dc. It is an iota high voltage unit. We ordered an mr2 (electric) steering pump for the power steering. It should be here shortly. We need to order a couple of pumps for the cooling loop and some vaccum hose and some liquid line. That should be about it aside from some wire and electrical connectors that will come up as we get things put together.
Aaron
I realized I got side tracked during our conversation and we never completed discussing the DC/DC. Our preference would be to use the Iota 55amp 220V Converter as a DC/DC. I have a new one handy, and
the size is about right to fit near the 12v battery up front. If you want to explore other options just let us know. I don't have much data on using the Vicor modules, so any pointers would be helpful.
Mark
The AC normally is pretty easy with a "close clutch"signal from the climate system. Right now that signal never appears to be triggered. My guess is the climate computer won't try to turn on the AC clutch unless the engine is idling, maybe? Seems odd, the signal doesn't go from the climate system straight to the clutch, it
passed thru the Engine computer which decides when to close the clutch. One would think the ECU could disable the clutch if the engine wasn't running. There is two signals going to the hot/cold mixing valve that we might be able to use instead, but it means disassembling the dash enough to tap that signal... not fun.
Alternative might be to put a tiny three position switch on the dash for the heater/AC or off.
Mark
Finally figured out the AC control circuitry. I've been trying to figure out why the climate control computer never asserted the signal line asking for the AC compressor to be turned on even when set to max
cooling. My suspicion was that it was waiting to see the engine running, and tonight I figured out a way to fake an idle signal to the dash using a speed sensor and my cordless drill.
The good news is that it works, once it sees the engine idling for about 30 seconds it starts pulsing the AC demand line.
Bad news... the pulsing won't turn on the Masterflux compressor, I need some circuitry to convert the pulsing signal to a continuous on signal for the masterflux controller. Other bad news... our motor doesn't idle, so the AC won't turn on until your going down the road and might turn off at traffic lights. So we probably need to build a circuit that sends a false "idling" tach signal to the dash when the motor is stopped. I'm going to ask Otmar if he might be able to add this to the Zilla software, it would be really easy for him. Otherwise I have to design a circuit and send out to have the PC board made. (Time consuming)
Mark
--------------------------------------------------------------------------
If it is a 205V pack, and we can output a max of 960A, I am wondering what the motor will actually see on early acceleration? Can the controller convert extra voltage not used at lower motor rpms into amps for the motor???
Rob
The large inductance of the motor and the switching of the controller produces what electrical engineers call a "buck" DC/DC converter. A Buck converter can step output voltage down and output current up. At low speeds the motor only needs 80-100V, so the output or motor amps might be double input (battery) amps. But the controller's 1000 or 2000amp rating is on the motor side. (Mostly the freewheel diodes, which work the hardest at slow speeds) As speed increases the motor wants to become a generator, so the motor side voltage has to go higher and higher to force amps into the motor and amps start falling off. Top speed is limited by how much voltage you can use to stuff
So in general, a 1000amp motor controller will never draw 1000amps on the battery side. At the most it will draw about 800, and usually more like 600-700.
A 2000 amp controller could draw up to 1200-1500 battery amps... but there are few batteries out there that can produce that for more than a second or two.
Mark
--------------------------------------------------------------------------
According to the 10C discharge curve on the web site I sent you, a mid-way discharged pack will drop from 3.2V to 2.4V under 10C load which is actually a 25% sag. That would give us 153V at presumably a full 960A. If we are using 100V, then we convert the extra 53V into a relative increase in Amps, which may be an additional 333A. So total, at 100V, we have 1293A to the motor, even more at lower speeds. Based on this concept, at low RPM, could we give the full 2000A capability of the Z2K to the motor with our pack? Acceleration will taper off as motor rpms increase due to higher voltage diminishing amp capability of the controller.
Rob
Its very complex, and difficult to model. The exact curve the batteries give will be a big factor. 20% sag may be optimistic, though these cells might surprise us. Since we have the option to try it first, lets see what the the difference is. We could even put the car on a dyno with each controller and see if there is a difference.
It may also be that a 1k produces the acceleration you want.
Mark
Early Contact with REVOLT
Quest for an Electric Porsche Boxster.
Select email exchanges between members of REVOLT Custom Electric during the conversion process:
3/15/2008
I am interested. I will be needing a new car in about a year. About how much "ballpark" would you estimate the cost of converting a decent luxury sedan or sports coupe? I am thinking along the lines of a Porsche, something light and sporty, perhaps a BMW roadster. I would like to find a great deal shopping for the donor car. I would be open to suggestions. Range will need to be at least a consistent 60 miles/chg up to at least 80 mph. I would like to consider the AC Propulsion system as well.
Rob
For a basic conversion, start with about $13,500 in parts including converted air conditioning, plus batteries and labor (I'll get to these in a moment). This will give you a decent, if not thrilling level of performance. For high street performance and/or racing applications, costs go up, but there are a few cost "thresholds" beyond which significant performance increases can be had for little additional money. While range can be a physical limitation, power and top speed are limited in a practical sense only by your budget.
For your desired range, and most likely the handling level you'll expect from a Porsche or BMW, you will want to use lithium batteries, which unfortunately carry a price premium today. The more economical choice is lead-acid, but it is very difficult to design in more than 45-50 miles of range with these large, heavy batteries. A pack of decent ones will cost you about $2500-3500 and typical range is 35-45 miles.
Currently our prices for lithium batteries (safe, non-burning lithium iron phosphate) are around 5-8 times that of lead-acid per watt-hour of storage. So that same 35-45 miles of range will cost you about $15,000, will weigh less than half and take up much less space, and will last at least twice as long. They'll also be immune to the sagging performance of lead-acid in the cold, and will remain brisk and powerful until they're empty due to their flat voltage/discharge curve.
At 65mph, I would roughly estimate a midlevel luxury sedan like a BMW 5 series would use about 350 watt-hours per mile at 65mph. Smaller cars (BMW Z4,Z8, P. Boxster) can be from 250-350, and of course midsize SUVs can be well over 400. At the moment, our lithium battery prices are right at $1 per watt hour, fully configured with battery management systems (electronic circuitry that watches and protects the batteries from abuse). You can figure your battery budget and desired range from that. (There are complications in the math with lead-acid batteries, but lithium is pretty simple). Drag-inducing add-ons like ground effects and spoilers can reduce your range, so many sleek-looking sports cars can actually have poorer efficiency than you'd expect at high speeds, but of course these features can be modified and there are other aero improvements (belly plate, grille closure) that can help.
DC systems offer comparatively high horsepower per dollar. This makes them suitable for budget builds, and cramming as much horsepower as possible for racing applications. Drag racing especially benefits from DC.
AC systems can be powerful, are generally more sophisticated and as it turns out are particularly well suited to autocross, but the economics are different since the design requires many more discrete silicon power switching transistors, one full "H-bridge" of transistors per phase (typical drive systems are 3-phase). A DC system in contrast has only 1 single switch.
Comparing drive units as directly as we can at the "high end": while a top-of-the-line DC controller for racing runs around $5k and puts out around 350HP (competitive with 500+HP in a gas engine) and a good dual motor configuration will set you back about 4k, plus DC/DC converter and 12kW charger for another 4k or so, in contrast a 200HP AC Propulsion AC150 drive package (which contains all those parts) costs about $27k plus about $1k for the coupling (ACP wants to supply the couplings for us for the time being instead of having us build them, they're pretty adamant about it).
AC has its benefits. Regenerative braking is almost nonexistent in DC controllers, but it's a standard feature in AC drives. It will get you about 10-15% additional range in stop-and-go and hilly driving. Just as important, it will make your brakes last a very long time (often as long as the car) and will keep brake dust off your wheels. Finally, the AC150 specifically has one really cool feature -- the rotor is stable up to 13,300 rpm. You can't shift a transmission with this motor unfortunately, but with that speed range you don't need to and you won't want to. You can leave it in 2nd gear, remove the shifter, and get fantastic torque at the wheels, uninterrupted, all the way to redline.
For other examples of converted EVs around the world, our nonprofit organization (AustinEV) hosts a site for the benefit of the EV community:
http://evalbum.com
Chris
--------------------------------------------------------------------------
I looked at Edmunds and I think a Z3 roadster might be a great, light chassis for the buck...
http://www.edmunds.com/used/1997/bmw/z3/724/specs.html#
I was going to suggest the Z8 (http://www.edmunds.com/used/2003/bmw/z8/100187551/specs.html), but it turns out these all-aluminum cars go for $70-90k, if you can find one :o)
I really like the S2000 (I'm a big honda fan). Compared to the Z3 it's a little longer, a tiny bit shorter vertically, a little wider, and a bit heavier:
http://www.edmunds.com/used/2007/honda/s2000/100800929/specs.html
It also has a much more powerful engine, meaning the transmission is more likely to be stout enough to handle a decent electric motor.
Another thought is the RX8, a much heavier car but with true seating for 4:
http://www.edmunds.com/used/2007/mazda/rx8/100793780/specs.html
The Mercedes SLKs are pretty nice, though you'd have to go back a few years to find one at a similar price, and they're also a bit heavy to start out with:
http://www.edmunds.com/used/2004/mercedesbenz/slkclass/100331689/specs.html
Like the 300Z before it, the 350Z is a very heavy car for its size:
http://www.edmunds.com/used/2007/nissan/350z/100846273/specs.html
But you also have a larger, heavier V6 engine to pull out of it. Still, I think the only reason to go with this one is if you have an particular liking for the style.
The Boxster is actually a lot lighter than I'd thought .. and of course it's received rave reviews from just about everyone. You do have to go back a few years to find one in the same $20k price category though:
http://www.edmunds.com/used/2003/porsche/boxster/100183117/specs.html
Of these, more popular cars like the RX8 or the S2000 may have less expensive upgrade parts (suspension, etc) than the european imports.
What are your thoughts on these so far?
Chris
--------------------------------------------------------------------------
Of these cars, the best deal will probably be a 98 Boxter or 97-98 Mercedes SLK convertible, probably favoring the Mercedes. I like the hardtop convertible idea if it is reliable. BMW 3 series is another possibility. I will look into both cars over the weekend. The other cars are too steeply priced. I think the maximum cost of the whole project including the donor car needs to be closer to $45-50k for me to stomach the risk.
Rob
--------------------------------------------------------------------------
http://visforvoltage.org/forum/2702-hands-test-48-volt-20-ah-lifepo4-pack-ping-battery
http://www.diyelectriccar.com/forums/showthread.php/evdl-best-bang-buck-battery-14029.html
http://www.diyelectriccar.com/forums/showthread.php/lithium-vs-lead-great-cost-debate-14035p5.html
Later in the above thread, Thunder sky may have a more reliable LiFePO4 option available now... Would it be sacrilege to get a bid from them for our project?
http://www.diyelectriccar.com/forums/showthread.php/lithium-vs-lead-great-cost-debate-14035p5.html
http://www.diyelectriccar.com/forums/showthread.php/lifepo4-group-purchase-14074p5.html
http://www.evpower.com.au/-PROJECTS-.html
Rob
--------------------------------------------------------------------------
The problem with these is that they're not really EV-duty cells. Given that "C" is a proportion of current from a battery to its capacity such that a 20Ah battery has a C value of 20, these cells are rated at 1C continuous discharge, with a burst to 3C. Our cells are rated at 4C continuous with a burst of 10C for 30 seconds, and 20C for 10 seconds. (The higher power cells I was referring to can do 50C in a 10 second burst, great for drag racing.)
You could make an EV pack out of these bicycle packs, but it wouldn't be very powerful, and worse, it would be operating at very close to the edge of its performance envelope, which will shorten its life. The further your normal operation is from the cells' maximum capability, the longer they'll last and the cooler they'll be in operation.
Looking at it another way, here's how you'd figure your pack with these modules. Each one is 36V and 20Ah. To reach a desirable target voltage of 200V, you'd need either 5 or 6. Let's choose 6 for now, so our voltage is 216V. A single string of these modules at 216V will contain 4.3kWh, so if you're looking for 16kWh, you'll need 4 in parallel. This will give you 80Ah, so a total of 17.28kWh of storage.
However, you'll only be able to deliver 240A of current, at a peak (and they don't mention how long that peak is). The 1K Zilla is capable of 800A of input, and so much of its capability (and the capability of your motor) will be wasted. Nominal current will only be 80A, which at 216V is 17.28kW continuous, or about 23HP. Take out about 10% power conversion plus drivetrain loss, and ultimately a flat level cruise at 65mph (approx 20HP) will again put you close to your pack's continuous current limit.
If you rearrange the voltage and current (higher or lower pack voltage, corresponding lower or higher current), the numbers will work out similarly.
Chris
--------------------------------------------------------------------------
We think the 9 should be sufficient for what you are trying to do. The 11 is heavier and more expensive. We might want to get you to take a ride in Mark's MR2 when he gets back from his trip on the 10th so that you can feel the acceleration there. He has a 9" motor in there at the moment with a z2k limited to 1000 amps so it would be similar to what this setup would do. If you want to get the 11 it would be $2325 instead of $1450 for the Warp 9.
From our calculations, we are thinking that a roughly 20 kWhr pack would get you what you want with the boxster. The price from K2 includes the raw cell cost (1920 cells) and the module assembly (48 modules). The total blade (module) cost from K2 is $15,468.00. There would also be shipping involved from China to them for module assembly and then to us that they estimate at about $600 or so. The BMS only adds $30-40 per module. We will have a better idea as to the exact cost of this once we are done with the Mazda.
The AC is in there. It is the Masterflux Sierra compressor. We have achieved outlet temps of around 53 degrees with the Saturn install with this compressor. That was at the low end of the range for the controller and your pack voltage would be higher. We will be testing the Saturn to see if the voltage to the controler matters. Masterflux is producing a higher capacity compressor (the Alpine) for release in mid 2009 and we can get an engineering sample (for around $1000 vs around $475) if you end up being unhappy with the Sierra's capacity.
Aaron
Oh, I meant to say that we are having K2 quote us on their 26650EV cells for these modules. We have a high enough voltage and several series strings in parallel so we are confident that we don't need to call for their more expensive performance EV cells.
Aaron
If you end up choosing the 911 we may need to redo the pack requirement a bit. I think those are a bit heavier. Though, a hard top would probably be more aerodynamic.
This pack size should do fine for performance. The voltage is around 200 and we are paralleling modules so we don't have to pull as many amps from each individual cell to feed the motor controller. If you want more range, we would need to add additional strings. If you end up wanting a small incremental increase in performance, the limiter would be the controller and the motor. Upgrading to a Z2K for example would add an additional $2000. I think the 9 and the Z1K coupled with the current pack would provide a very fun, high performance car, but if you want blistering performance (REALLY want the drag race type horse power), we may want to consider going with the high performance cells from K2, the Z2k and the Warp 11. That will be more expensive.
I'll write up a description of the 9 vs 11 tradeoffs for you when I get back in from the garage this afternoon.
Aaron
--------------------------------------------------------------------------
My reading last night indicates the Warp9 is limited to a Max of less than 192V before you risk flash damage. So current will be the limit for max power, 192 kW max, probably less to the motor. I think the continuous power from the controller will be closer to 300 amps with Z1K, so continuous power will be closer to 50 kW). I think we want the Boxster to have at least the equivalent power of its stock engine. I want the car to be as peppy as a stock boxster, maybe a little peppier. We should shoot for 0-60 in less than 6 secs and sustain 85 mph without overheating the controller. That is not drag racing, but may require the upgrade. Hopefully you guys can figure this out, because I am not sure what will work and reading about it is confusing at best.
Rob
Actually, on the battery side, the maximum input current to the Z1K controller is 800A. A 200V nominal pack will sag under this much current, so peak input into the controller is going to be somewhere around 150kW or so. Figuring for controller, motor and drivetrain losses, this equates to about 150 peak HP at the wheels (rough estimate).
However, even though it seems like we have a recognizable figure (peak HP), we're still not really comparing apples to apples. I think you'll find this level of performance to "feel" as strong or more so than the 230HP engine in a stock 2004 Boxster (for example), and the reason for this is torque. Horsepower does not push the car; torque is the force that causes acceleration. Peak horsepower really only describes the extent of velocity at which you can still expect that accelerating force to be available. At freeway speeds you should be well within the envelope in which torque will be more important than peak horsepower.
The stock 1997 Boxster engine produced 180 ft-lbs of torque; the 2004 produced 192. The 2004 Boxster S topped out at 250. At 1000A, a Warp 9 should easily be able to do 250-300 ft-lbs. AND it will do so, *not* at a peak only achievable within a narrow RPM band. With an electric motor, that peak torque is available from zero RPM and holds flat until back-EMF from the motor reaches the vicinity of battery voltage as speed increases. This one factor makes a huge difference. More area under the torque curve = better overall acceleration with less peak horsepower. You will want an upgraded clutch, as even on sportscars, stock clutches tend to slip.
With our recommended configuration, it's true you will most likely not be able to continue accelerating as briskly as stock at higher speeds. A 200V pack will still be fun when you mash it at 55mph, but it's not going to be quite as thrilling at 70. If you really do want to be able to keep up with a stock Boxster when accelerating beyond the speed limit, then you will want at least the larger controller. Of course as a bonus, you'll then be able to easily embarrass the stock Boxster, and most other cars on the road, at lower speeds with essentially double the available torque. You'll also be at significant risk of damaging your transmission and driveline, if you can maintain traction. A multi-disk racing clutch will be a must.
I think the continuous power from the controller will be closer to 300 amps with Z1K, so continuous power will be closer to 50 kW).
If your Boxster requires any more than 30KW or so to maintain 70mph, I'll be surprised. Continuous horsepower is only important in making sure you can cruise on the highway without heat continuing to build up in the motor. It will definitely be limited more by the motor than by the controller, and a motor's continuous horsepower rating can be increased by increasing airflow, either by ram-air or a blower, or both. The Warp11 does have a higher continuous horsepower than the Warp9, but if you already have more than you need to maintain cruising speed, then the larger motor doesn't help for this purpose. Sustained cruising at very high speed may justify a Warp11, but it will also significantly impact your driving range. (Horsepower to overcome aero drag increases proportional to velocity cubed.)
BTW, continuous power is also something that's routinely ignored by some people in building lead-acid powered cars, since lead-acid typically doesn't provide enough range to overheat a mildly overloaded motor. Driving for only 30 minutes at a time allows you to exceed the motor's rating, and get away with it.
I think we want the Boxster to have at least the equivalent power of its stock engine. I want the car to be as peppy as a stock boxster, maybe a little peppier.
Remember, peak horsepower is only meaningful when needing acceleration at high speeds. Comparing gasoline-powered cars, people toss around peak horsepower as a number that more-or-less represents how well the car will accelerate. Even though some cars have better torque than others, the HP figure is meaningful for comparison because gas engines have roughly similar torque curves. An electric motor is radically different, so you have to abandon the convenient number and get into more detail to get an idea of how the car will behave.
I would be *thrilled* to put a Warp11, a Z2k and 200V of lithium in your car. It would turn your project into an important demonstration of our services, and ultimately it's exactly the sort of project we'd like to focus on as our core business in the future. You'll have more peak horsepower than stock for fantastic acceleration at high speeds, and at legal speeds your car will have entry-level supercar performance. Your car will also be heavier, less efficient, and more likely to break something when driven hard, since you'll be so far outside the drivetrain's torque specifications. Netgain has already received reports specifically about Boxsters breaking axles with a Warp 11 at 2000 amps.
I've been recommending the Z1K and Warp9 because it seems the most in line with your stated goals and intentions for the car (or as far as I've understood them), and because the 11-inch motor is twice the price for far less than double the torque per amp of current. Ultimately it's your choice, and I hope we can gather enough information that it will be reasonably well-informed.
We should shoot for 0-60 in less than 6 secs and sustain 85 mph without overheating the controller. That is not drag racing, but may require the upgrade. Hopefully you guys can figure this out, because I am not sure what will work and reading about it is confusing at best.
I think we may be able to achieve close to 6 seconds with 200V, 1000A and a Warp 9. I'll see if I can get some numbers to substantiate that; it's notoriously difficult because Netgain does not publish performance details for their motors at high voltages and current. (I talked at length with Netgain yesterday and have asked for some specific performance information that they're going to try to get for me.)
If you would like to maintain 85mph continuously and your car is based on lithium, a Warp 11 and a Z2K may indeed be the safer bet; the components will certainly run cooler. However, your range will be disappointing, and again if your car is based on lead-acid batteries, you probably won't be driving long enough to build up enough heat to damage the lesser components anyway.
Torque and power are definitely confusing subjects, even more when you realize that the way we use these figures in conversation about cars is based on assumptions that no longer hold true when dealing with such a different animal as an EV, especially one based on a series-wound DC motor. I hope my attempts to explain haven't made the water even muddier than before.
Chris
--------------------------------------------------------------------------
After thinking about our discussion yesterday, we might want to look at using the Warp11 with the Z1K. It will be easy to upgrade the Z1K later, but not the Warp 11. The Warp 11 may give better durability, run cooler, and gives more efficiency per amp. Granted it weighs more which might offset those benefits, but 70lbs extra doesn't seem like a lot in a 3000lb car. Looking at the torque curves makes it look like it gets about 10% more torque per amp from the battery. Thermodynamically, this makes sense as it produces less heat and must be converting the energy into mechanical power.
If the torque per amp is 10% more than the Warp 9, and the cost of the upgrade is much less than 10% of the total project, then it makes sense to go with the Warp 11. Of course I am just guessing about the math at this point. You would think Netgain would have this all over their website comparing these motors price points, etc.
Rob
--------------------------------------------------------------------------
I seem to remember that the 11" is not very efficient at high current due to large end turn ratio, so I would be leaning toward the 9". But the Boxter is a tank at 3000# and so will be a challenge. Much will depend on the voltage and power capability of the batteries.
You may be able to hit six seconds with a Z1K and a high voltage pack, but in your case I would use the Z2K-HV or -EHV depending on your pack voltage.
I don't know the wait time, Mandi can get back to you on that.
--
I hope this helps,
-Otmar
--------------------------------------------------------------------------
Just wanted to clarify about the motor efficiency comparison. The short answer is, yes the Warp 11 is more efficient than the Warp 9, enough so that it may be justification enough to go with it, especially if your driving habits don't involve a lot of in-town driving where the motor's higher static and dynamic masses will impact efficiency in ways not accounted for in the official specs.
The *reasons* why the Warp 11 is more efficient are a bit different though. In fact, in most cases a larger motor is *less* efficient than a small one, for a few reasons. First and foremost, a larger motor has a larger brush contact area, stiffer brush springs and a larger commutator diameter. This creates a higher continuous friction loss, and the difference is easy to feel. The motor is physically harder to turn by hand. It also has a larger fan which moves more air; this creates a higher windage loss at high RPMs.
Second, while it is true that a larger motor produces more torque per amp, there's no free lunch, and "torque" and "amps" are only components of total power, not the whole picture. If there were nothing else to the equation, a massive dishwasher-sized motor making vastly more torque per amp would somehow magically produce more shaft power on the output than the electrical power you put into it. The reality is that there is a tradeoff -- "back-EMF" (voltage drop across the motor) rises with RPM faster in a larger motor. To cause the motor to draw a given current at a given shaft speed, the controller has to supply proportionately more voltage to the larger motor. Thus, the power input (volts x amps) grows to match the power output (ft-lbs x rpm). Greater torque from the larger motor has zero effect on efficiency.
Third, while a larger motor may run cooler at a given power throughput, this is in part because it has a larger surface area (internally and externally) from which to dissipate heat. Though *temperature* may not climb as high (and this is good in terms of avoiding overheat and insulation failure), total heat dissipation may be as high or higher in a larger motor. "Heat" is a measure of thermal power, "temperature" is only one component of heat.
The Warp11 is indeed more efficient than the Warp9, because the 11's design was more thoroughly dictated by Netgain with fewer concessions to any existing Warfield design. It's simply a better-designed motor for EV use, though the difference is not earth-shattering. The Warp 9 is primarily an industrial motor, with a few changes to make it physically compatible with the 9" Advanced DC motor, and a few other changes to make it better able to handle an automotive operating environment. For what it's worth, Netgain has been working on producing a variant of the Warp 9 that incorporates some true EV-oriented modifications, and they actually have a prototype now but it will not be available on the market for a few months at least. For those who aren't racing, we usually recommend the Warp 9 to save money on capacity that's not needed, and because in heavy stop-and-go driving the extra weight may more than cancel the 11's inherent efficiency advantage in steady-state operation.
Be aware that you're not just adding 70lbs to the car. Some of this is static mass, but some of it is spinning mass, which implies a much higher impact because you need to not only accelerate it forward, you also have to get it spinning; this is why we usually lighten flywheels when we're doing a conversion. All the energy required to spin up the heavier armature is lost as heat during braking and upshifting.
I've had several conversations with Netgain about documentation and making information available to customers, or at least to their vendors. They realize they have a problem, and are working to address it.
Again, I would be totally happy to install a Warp11 in the Boxster, and leave enough room in the design for a future Z2K upgrade. I'd be thrilled to show off what we're doing, and I think it would make a very impressive statement about the sort of conversion work we can do. As an added advantage to us, Netgain will be able to ship an 11 to us a month sooner than a 9", due to their production schedule (9s won't be available until October). I just want to make sure you're making this decision on the basis of accurate information and realistic expectations.
Chris
--------------------------------------------------------------------------
I really don't know what to do on this one. So, we should probably go with the most experienced opinion in the group, which ain't mine. Otmar seems to favor the Warp 9 and maybe going with a Z2K. But in his reply, he acts like he doesn't know the max voltage on these motors is 172-192V. Why else would he offer up the Z2K EHV?
Did you guys have any pull in getting the controller faster? Aaron said he thought Nov was the delivery date, but that email says 6mos!
Rob
--------------------------------------------------------------------------
Be aware that battery voltage does not equal motor voltage, and the battery voltage limit does not need to be influenced by the motor voltage limit, with the exception that it cannot be higher.
So, it would be perfectly reasonable to set a 175V limit on motor voltage for example, and run the controller from a 200V or 300V, or even 350V pack. The additional voltage gives the designer flexibility in adding more energy capacity, by simply making the string longer. That flexibility isn't as important with a lithium pack where the cells are smaller and module design is more flexible, but it still can be helpful.
Also, be aware that Otmar is a racer and a performance junkie -- like I am, but with a greater tendency to let it influence his recommendations. :o) He's right; you may or may not be able to do 0-60 in six seconds. But it will be close, 0-30 is going to be awesome, and the rest of the way up isn't going to be too bad.
One thing is for certain; Zillas are scarce and will continue to be for an unforeseeable amount of time. This means they maintain their resale value really well, should you choose to buy a Z1K now and upgrade in the future.
Chris
--------------------------------------------------------------------------
I thought we had all this worked out, but it keeps grating me to do more research... Looks like we will be going with a Z2K HV, and it will be available closer to schedule. But the motor decision is still difficult.
Based on everything you are telling me, and everything I have read, I have made some rough conclusions (possibly wrong):
The Warp 11 will be much easier to damage the transmission and clutch, but give drag racing performance off the line. It will then lose power and acceleration at freeway speeds. It will use more battery to maintain higher speeds if higher rpms are required, thus less range. But could gear shifting overcome this?
The Warp 9 will be less hard on the tranny and still go very fast off the line. It will also accelerate faster at higher rpms and possibly have the fastest top speed and longer range.
I guess if I had to choose priorities, I would like to not destroy the car, require expensive upgrades, or easily lose control and wreck the car and/or myself. I would prefer to accelerate faster from 30 to 70, than from 0 -30. I would like the car to be more efficient at 70 mph if one motor is superior to the other at 70mph constant speed.
I am thinking the Warp 9 will be the better choice, especially with Otmars comments. I did see some other forum comments about the Warp 11 using more amps compared to the Warp9 at highway speeds. If that is true, we should probably use the Warp9. I find it troubling how little commentary there is out there about this application and what works best. In fact on the evalbum pages, there are few finished cars using either a single Warp9 or Warp11 with a Zilla which quote great performance, typically using lead batteries. I see several using two 9s with Z2k, but they are trying to compete at the 1/4 mile.
Best wishes,
Rob
--------------------------------------------------------------------------
All series-wound DC electric drive systems (motor + controller) exhibit the characteristic of torque output that remains relatively constant until a certain RPM, after which torque decreases with increasing speed. That RPM "knee in the graph" is partly dependent on the motor, but mostly dependent on battery voltage. Both the 9 and the 11, and any other series DC motor behaves this way. The numbers are obviously different, but the curves are the same shape. All electric drive systems will exhibit better performance at low speeds, because below the RPM threshold the controller will be able to deliver maximum motor current.
Under 2000A, the Warp9 still poses a threat to your driveline, though not as significant as the Warp11.
The Warp9 will not necessarily provide better performance at high speed, unless you have insufficient battery voltage to continue driving the larger motor at the same current. Provided your battery voltage is high enough (200V should be), the opposite will be true.
You don't need to worry much about "damaging" your clutch; the main concern with the clutch is the nuisance of realizing that it's slipping, and needing to dismantle everything and upgrade it. This is why we specify an upgraded clutch at 1000A, and a racing clutch at 2000A.
Neither motor puts you at an unusual risk of losing control of the car, over and above the risk inherent with any powerful sports car. With 2000 motor amps either motor will provide enough torque to break traction. This is not special or unique to electric motors and as with any high performance rear-wheel-drive car, care must be taken not to apply too much power around corners without the skill to execute a controlled drift. Due to the inertia of the armature this will "feel" a bit different in an electric and will require some getting used to. Remember, you have a finite amount of traction available, and the force vectors for propulsion/braking and cornering add together to approach that limit. For example if you're coasting around a corner at the very edge of your tires' traction, any throttle at all will break the rear end loose, and vice versa.
Here's my take on the situation: In an ideal world my recommendation of 1000A and a Warp9 stands; though it may not quite deliver 0-60 in 6 seconds it certainly would come very close and would provide very entertaining performance on the street and highway. We have the problem of delivery date for the controller which has caught us by surprise, and so you've decided to pay the extra for the Z2K. You will certainly get what you're paying for, and this upgrade alone is going to give you incredible performance regardless of which motor you choose. Again, the power of the car is determined far more by the controller and batteries than the motor.
So between the Warp11 and the Warp9, the 11 will give you a modest amount of additional torque (which you don't need), and will give you higher continuous horsepower (which might be helpful if you like to cruise at 85mph). Netgain's dyno test data shows the Warp11 to be a little more efficient in steady-state operation, though they have not been able to test it across its entire performance envelope and you have seen reports from others that seem to contradict their data.
If it were me, I'd get the Warp11 (we'll note I bought a Warp13 for my truck project).
On the other hand if we're talking about a sane customer not interested in racing, I'd get the Warp9 and save some money. I think it's a given we'll be force-cooling it with a blower, so it will have a little higher continuous horsepower than stock anyway.
(For what it's worth, we wouldn't have to force cool the 11, so a little less complexity, noise and energy consumption there.)
Either way, with the Z2K, the car is going to rock. Perhaps a little too much for its own good, but we do have the ability to detune the controller's output, to a "normal driving" profile that you can change to a "pull out the stops" profile at the flip of a switch.
--
Chris
--------------------------------------------------------------------------
The difference in efficiency (and therefore range) with mixed driving will not be significant. It's hard to tell which would be better; both motors have pros and cons that would mostly cancel each other out. I believe the difference will not be sufficient to consider this an important factor.
Dual motors is a setup that's used by racers to improve horsepower. Though it seems to be something that would appeal to you, I've been hesitant to bring it up for a few reasons. Mainly, you're venturing completely into all-out electric supercar territory here, with more torque from two 8" motors than a single 11". You have the complexities of the mounting arrangement (John Wayland spent years and several revised designs trying to work out the vibration problems in his dual-motor setup, and eventually had the two motors fused together into one unit by custom motor builder Jim Husted.)
Second, to take advantage of the extra power, you'd need to raise your pack voltage, to at least 250-300V. You'd also need the series/parallel switching option on the Zilla, and the extra contactors and traction wiring that this implies.
As Aaron mentioned, this is usually the sort of setup that gets connected directly to the differential without the use of a transmission, simply because of the problems with putting that much torque through a transmission, and the absence of even high end multi-disk racing clutches that will operate without slipping. Unfortunately in your situation I think that all Boxster transmissions are of the traditional VW layout; the differential is integrated and cannot be used separately. Converting to a solid axle rear for direct drive would be an extreme undertaking, and would have effects on handling. Otmar dealt with the situation by retaining his transmission despite the use of two 8" motors, and his solution is to swap between 2 transmissions. One gets rebuilt while the other is in use.
On the other hand, dual motors would indeed get you higher horsepower at the top end. Essentially, you provide more output voltage than a single motor can stand by itself, but the voltage is split between the two motors because they're wired in series. As you speed up, the torque remains flat until a certain point, and it starts to drop off. When the Zilla calculates that it's the optimum time to do so, it cuts power for a fraction of a second, rearranges the motors electrically into a parallel configuration, and then each motor is fed full pack voltage. At high speed the motor can theoretically tolerate a higher voltage than it's rated for, and so suddenly you get a burst of additional torque. Sometimes a motor will flash over when treated like this; it's pretty unpredictable and is a risk inherent in the racing scenario for which this approach was designed. The flashover risk can be mitigated by variable brush timing, though this implies custom work on the motor (until Netgain releases their new Warp9 with built-in variable timing) and a control system for which no standard off-the-shelf products yet exist.
Another attempt at a summary:
Get the 9, save some money, have a lighter car that will handle a little better on corners due to a somewhat lighter rear end. Enjoy potentially better efficiency while accelerating, for in-town driving. Deal with some minor blower noise (which will be most obvious at a stop, when the air conditioning fans aren't running).
Or, if the extra cost isn't a showstopper, get the 11, have a bit more power and a motor that will run cooler and (according to Netgain's data) more efficiently when operated continuously on the highway. With no forced cooling necessary, the install will be simpler. Deal with some added weight.
Chris
--------------------------------------------------------------------------
Here are a couple of good web sites on K2 and A123
http://www.zeva.com.au/tech/K2/
http://zeva.com.au/A123/
I corresponded with Ian Hooper (the author of these reviews) last year on the subject of his testing, and his findings were definitely influential in our decision to go with K2 as our lithium supplier. We've built what I think is a great and promising relationship with them so far, and are in the process of negotiating some arrangements to move more volume and lower our prices in the future. Having visited their headquarters in Nevada and having seen the verification they perform on every cell during module assembly, seeing their heated and chilled enclosures for long term temperature-controlled testing, and knowing that they're actually licensed to legally sell LiFePO4 cells in the US (A123 is not licensed, and is currently being sued by Phostech and the University of Texas); I feel confident we've made the right decision.
Chris
Select email exchanges between members of REVOLT Custom Electric during the conversion process:
3/15/2008
I am interested. I will be needing a new car in about a year. About how much "ballpark" would you estimate the cost of converting a decent luxury sedan or sports coupe? I am thinking along the lines of a Porsche, something light and sporty, perhaps a BMW roadster. I would like to find a great deal shopping for the donor car. I would be open to suggestions. Range will need to be at least a consistent 60 miles/chg up to at least 80 mph. I would like to consider the AC Propulsion system as well.
Rob
For a basic conversion, start with about $13,500 in parts including converted air conditioning, plus batteries and labor (I'll get to these in a moment). This will give you a decent, if not thrilling level of performance. For high street performance and/or racing applications, costs go up, but there are a few cost "thresholds" beyond which significant performance increases can be had for little additional money. While range can be a physical limitation, power and top speed are limited in a practical sense only by your budget.
For your desired range, and most likely the handling level you'll expect from a Porsche or BMW, you will want to use lithium batteries, which unfortunately carry a price premium today. The more economical choice is lead-acid, but it is very difficult to design in more than 45-50 miles of range with these large, heavy batteries. A pack of decent ones will cost you about $2500-3500 and typical range is 35-45 miles.
Currently our prices for lithium batteries (safe, non-burning lithium iron phosphate) are around 5-8 times that of lead-acid per watt-hour of storage. So that same 35-45 miles of range will cost you about $15,000, will weigh less than half and take up much less space, and will last at least twice as long. They'll also be immune to the sagging performance of lead-acid in the cold, and will remain brisk and powerful until they're empty due to their flat voltage/discharge curve.
At 65mph, I would roughly estimate a midlevel luxury sedan like a BMW 5 series would use about 350 watt-hours per mile at 65mph. Smaller cars (BMW Z4,Z8, P. Boxster) can be from 250-350, and of course midsize SUVs can be well over 400. At the moment, our lithium battery prices are right at $1 per watt hour, fully configured with battery management systems (electronic circuitry that watches and protects the batteries from abuse). You can figure your battery budget and desired range from that. (There are complications in the math with lead-acid batteries, but lithium is pretty simple). Drag-inducing add-ons like ground effects and spoilers can reduce your range, so many sleek-looking sports cars can actually have poorer efficiency than you'd expect at high speeds, but of course these features can be modified and there are other aero improvements (belly plate, grille closure) that can help.
DC systems offer comparatively high horsepower per dollar. This makes them suitable for budget builds, and cramming as much horsepower as possible for racing applications. Drag racing especially benefits from DC.
AC systems can be powerful, are generally more sophisticated and as it turns out are particularly well suited to autocross, but the economics are different since the design requires many more discrete silicon power switching transistors, one full "H-bridge" of transistors per phase (typical drive systems are 3-phase). A DC system in contrast has only 1 single switch.
Comparing drive units as directly as we can at the "high end": while a top-of-the-line DC controller for racing runs around $5k and puts out around 350HP (competitive with 500+HP in a gas engine) and a good dual motor configuration will set you back about 4k, plus DC/DC converter and 12kW charger for another 4k or so, in contrast a 200HP AC Propulsion AC150 drive package (which contains all those parts) costs about $27k plus about $1k for the coupling (ACP wants to supply the couplings for us for the time being instead of having us build them, they're pretty adamant about it).
AC has its benefits. Regenerative braking is almost nonexistent in DC controllers, but it's a standard feature in AC drives. It will get you about 10-15% additional range in stop-and-go and hilly driving. Just as important, it will make your brakes last a very long time (often as long as the car) and will keep brake dust off your wheels. Finally, the AC150 specifically has one really cool feature -- the rotor is stable up to 13,300 rpm. You can't shift a transmission with this motor unfortunately, but with that speed range you don't need to and you won't want to. You can leave it in 2nd gear, remove the shifter, and get fantastic torque at the wheels, uninterrupted, all the way to redline.
For other examples of converted EVs around the world, our nonprofit organization (AustinEV) hosts a site for the benefit of the EV community:
http://evalbum.com
Chris
--------------------------------------------------------------------------
I looked at Edmunds and I think a Z3 roadster might be a great, light chassis for the buck...
http://www.edmunds.com/used/1997/bmw/z3/724/specs.html#
I was going to suggest the Z8 (http://www.edmunds.com/used/2003/bmw/z8/100187551/specs.html), but it turns out these all-aluminum cars go for $70-90k, if you can find one :o)
I really like the S2000 (I'm a big honda fan). Compared to the Z3 it's a little longer, a tiny bit shorter vertically, a little wider, and a bit heavier:
http://www.edmunds.com/used/2007/honda/s2000/100800929/specs.html
It also has a much more powerful engine, meaning the transmission is more likely to be stout enough to handle a decent electric motor.
Another thought is the RX8, a much heavier car but with true seating for 4:
http://www.edmunds.com/used/2007/mazda/rx8/100793780/specs.html
The Mercedes SLKs are pretty nice, though you'd have to go back a few years to find one at a similar price, and they're also a bit heavy to start out with:
http://www.edmunds.com/used/2004/mercedesbenz/slkclass/100331689/specs.html
Like the 300Z before it, the 350Z is a very heavy car for its size:
http://www.edmunds.com/used/2007/nissan/350z/100846273/specs.html
But you also have a larger, heavier V6 engine to pull out of it. Still, I think the only reason to go with this one is if you have an particular liking for the style.
The Boxster is actually a lot lighter than I'd thought .. and of course it's received rave reviews from just about everyone. You do have to go back a few years to find one in the same $20k price category though:
http://www.edmunds.com/used/2003/porsche/boxster/100183117/specs.html
Of these, more popular cars like the RX8 or the S2000 may have less expensive upgrade parts (suspension, etc) than the european imports.
What are your thoughts on these so far?
Chris
--------------------------------------------------------------------------
Of these cars, the best deal will probably be a 98 Boxter or 97-98 Mercedes SLK convertible, probably favoring the Mercedes. I like the hardtop convertible idea if it is reliable. BMW 3 series is another possibility. I will look into both cars over the weekend. The other cars are too steeply priced. I think the maximum cost of the whole project including the donor car needs to be closer to $45-50k for me to stomach the risk.
Rob
--------------------------------------------------------------------------
http://visforvoltage.org/forum/2702-hands-test-48-volt-20-ah-lifepo4-pack-ping-battery
http://www.diyelectriccar.com/forums/showthread.php/evdl-best-bang-buck-battery-14029.html
http://www.diyelectriccar.com/forums/showthread.php/lithium-vs-lead-great-cost-debate-14035p5.html
Later in the above thread, Thunder sky may have a more reliable LiFePO4 option available now... Would it be sacrilege to get a bid from them for our project?
http://www.diyelectriccar.com/forums/showthread.php/lithium-vs-lead-great-cost-debate-14035p5.html
http://www.diyelectriccar.com/forums/showthread.php/lifepo4-group-purchase-14074p5.html
http://www.evpower.com.au/-PROJECTS-.html
Rob
--------------------------------------------------------------------------
The problem with these is that they're not really EV-duty cells. Given that "C" is a proportion of current from a battery to its capacity such that a 20Ah battery has a C value of 20, these cells are rated at 1C continuous discharge, with a burst to 3C. Our cells are rated at 4C continuous with a burst of 10C for 30 seconds, and 20C for 10 seconds. (The higher power cells I was referring to can do 50C in a 10 second burst, great for drag racing.)
You could make an EV pack out of these bicycle packs, but it wouldn't be very powerful, and worse, it would be operating at very close to the edge of its performance envelope, which will shorten its life. The further your normal operation is from the cells' maximum capability, the longer they'll last and the cooler they'll be in operation.
Looking at it another way, here's how you'd figure your pack with these modules. Each one is 36V and 20Ah. To reach a desirable target voltage of 200V, you'd need either 5 or 6. Let's choose 6 for now, so our voltage is 216V. A single string of these modules at 216V will contain 4.3kWh, so if you're looking for 16kWh, you'll need 4 in parallel. This will give you 80Ah, so a total of 17.28kWh of storage.
However, you'll only be able to deliver 240A of current, at a peak (and they don't mention how long that peak is). The 1K Zilla is capable of 800A of input, and so much of its capability (and the capability of your motor) will be wasted. Nominal current will only be 80A, which at 216V is 17.28kW continuous, or about 23HP. Take out about 10% power conversion plus drivetrain loss, and ultimately a flat level cruise at 65mph (approx 20HP) will again put you close to your pack's continuous current limit.
If you rearrange the voltage and current (higher or lower pack voltage, corresponding lower or higher current), the numbers will work out similarly.
Chris
--------------------------------------------------------------------------
We think the 9 should be sufficient for what you are trying to do. The 11 is heavier and more expensive. We might want to get you to take a ride in Mark's MR2 when he gets back from his trip on the 10th so that you can feel the acceleration there. He has a 9" motor in there at the moment with a z2k limited to 1000 amps so it would be similar to what this setup would do. If you want to get the 11 it would be $2325 instead of $1450 for the Warp 9.
From our calculations, we are thinking that a roughly 20 kWhr pack would get you what you want with the boxster. The price from K2 includes the raw cell cost (1920 cells) and the module assembly (48 modules). The total blade (module) cost from K2 is $15,468.00. There would also be shipping involved from China to them for module assembly and then to us that they estimate at about $600 or so. The BMS only adds $30-40 per module. We will have a better idea as to the exact cost of this once we are done with the Mazda.
The AC is in there. It is the Masterflux Sierra compressor. We have achieved outlet temps of around 53 degrees with the Saturn install with this compressor. That was at the low end of the range for the controller and your pack voltage would be higher. We will be testing the Saturn to see if the voltage to the controler matters. Masterflux is producing a higher capacity compressor (the Alpine) for release in mid 2009 and we can get an engineering sample (for around $1000 vs around $475) if you end up being unhappy with the Sierra's capacity.
Aaron
Oh, I meant to say that we are having K2 quote us on their 26650EV cells for these modules. We have a high enough voltage and several series strings in parallel so we are confident that we don't need to call for their more expensive performance EV cells.
Aaron
If you end up choosing the 911 we may need to redo the pack requirement a bit. I think those are a bit heavier. Though, a hard top would probably be more aerodynamic.
This pack size should do fine for performance. The voltage is around 200 and we are paralleling modules so we don't have to pull as many amps from each individual cell to feed the motor controller. If you want more range, we would need to add additional strings. If you end up wanting a small incremental increase in performance, the limiter would be the controller and the motor. Upgrading to a Z2K for example would add an additional $2000. I think the 9 and the Z1K coupled with the current pack would provide a very fun, high performance car, but if you want blistering performance (REALLY want the drag race type horse power), we may want to consider going with the high performance cells from K2, the Z2k and the Warp 11. That will be more expensive.
I'll write up a description of the 9 vs 11 tradeoffs for you when I get back in from the garage this afternoon.
Aaron
--------------------------------------------------------------------------
My reading last night indicates the Warp9 is limited to a Max of less than 192V before you risk flash damage. So current will be the limit for max power, 192 kW max, probably less to the motor. I think the continuous power from the controller will be closer to 300 amps with Z1K, so continuous power will be closer to 50 kW). I think we want the Boxster to have at least the equivalent power of its stock engine. I want the car to be as peppy as a stock boxster, maybe a little peppier. We should shoot for 0-60 in less than 6 secs and sustain 85 mph without overheating the controller. That is not drag racing, but may require the upgrade. Hopefully you guys can figure this out, because I am not sure what will work and reading about it is confusing at best.
Rob
Actually, on the battery side, the maximum input current to the Z1K controller is 800A. A 200V nominal pack will sag under this much current, so peak input into the controller is going to be somewhere around 150kW or so. Figuring for controller, motor and drivetrain losses, this equates to about 150 peak HP at the wheels (rough estimate).
However, even though it seems like we have a recognizable figure (peak HP), we're still not really comparing apples to apples. I think you'll find this level of performance to "feel" as strong or more so than the 230HP engine in a stock 2004 Boxster (for example), and the reason for this is torque. Horsepower does not push the car; torque is the force that causes acceleration. Peak horsepower really only describes the extent of velocity at which you can still expect that accelerating force to be available. At freeway speeds you should be well within the envelope in which torque will be more important than peak horsepower.
The stock 1997 Boxster engine produced 180 ft-lbs of torque; the 2004 produced 192. The 2004 Boxster S topped out at 250. At 1000A, a Warp 9 should easily be able to do 250-300 ft-lbs. AND it will do so, *not* at a peak only achievable within a narrow RPM band. With an electric motor, that peak torque is available from zero RPM and holds flat until back-EMF from the motor reaches the vicinity of battery voltage as speed increases. This one factor makes a huge difference. More area under the torque curve = better overall acceleration with less peak horsepower. You will want an upgraded clutch, as even on sportscars, stock clutches tend to slip.
With our recommended configuration, it's true you will most likely not be able to continue accelerating as briskly as stock at higher speeds. A 200V pack will still be fun when you mash it at 55mph, but it's not going to be quite as thrilling at 70. If you really do want to be able to keep up with a stock Boxster when accelerating beyond the speed limit, then you will want at least the larger controller. Of course as a bonus, you'll then be able to easily embarrass the stock Boxster, and most other cars on the road, at lower speeds with essentially double the available torque. You'll also be at significant risk of damaging your transmission and driveline, if you can maintain traction. A multi-disk racing clutch will be a must.
I think the continuous power from the controller will be closer to 300 amps with Z1K, so continuous power will be closer to 50 kW).
If your Boxster requires any more than 30KW or so to maintain 70mph, I'll be surprised. Continuous horsepower is only important in making sure you can cruise on the highway without heat continuing to build up in the motor. It will definitely be limited more by the motor than by the controller, and a motor's continuous horsepower rating can be increased by increasing airflow, either by ram-air or a blower, or both. The Warp11 does have a higher continuous horsepower than the Warp9, but if you already have more than you need to maintain cruising speed, then the larger motor doesn't help for this purpose. Sustained cruising at very high speed may justify a Warp11, but it will also significantly impact your driving range. (Horsepower to overcome aero drag increases proportional to velocity cubed.)
BTW, continuous power is also something that's routinely ignored by some people in building lead-acid powered cars, since lead-acid typically doesn't provide enough range to overheat a mildly overloaded motor. Driving for only 30 minutes at a time allows you to exceed the motor's rating, and get away with it.
I think we want the Boxster to have at least the equivalent power of its stock engine. I want the car to be as peppy as a stock boxster, maybe a little peppier.
Remember, peak horsepower is only meaningful when needing acceleration at high speeds. Comparing gasoline-powered cars, people toss around peak horsepower as a number that more-or-less represents how well the car will accelerate. Even though some cars have better torque than others, the HP figure is meaningful for comparison because gas engines have roughly similar torque curves. An electric motor is radically different, so you have to abandon the convenient number and get into more detail to get an idea of how the car will behave.
I would be *thrilled* to put a Warp11, a Z2k and 200V of lithium in your car. It would turn your project into an important demonstration of our services, and ultimately it's exactly the sort of project we'd like to focus on as our core business in the future. You'll have more peak horsepower than stock for fantastic acceleration at high speeds, and at legal speeds your car will have entry-level supercar performance. Your car will also be heavier, less efficient, and more likely to break something when driven hard, since you'll be so far outside the drivetrain's torque specifications. Netgain has already received reports specifically about Boxsters breaking axles with a Warp 11 at 2000 amps.
I've been recommending the Z1K and Warp9 because it seems the most in line with your stated goals and intentions for the car (or as far as I've understood them), and because the 11-inch motor is twice the price for far less than double the torque per amp of current. Ultimately it's your choice, and I hope we can gather enough information that it will be reasonably well-informed.
We should shoot for 0-60 in less than 6 secs and sustain 85 mph without overheating the controller. That is not drag racing, but may require the upgrade. Hopefully you guys can figure this out, because I am not sure what will work and reading about it is confusing at best.
I think we may be able to achieve close to 6 seconds with 200V, 1000A and a Warp 9. I'll see if I can get some numbers to substantiate that; it's notoriously difficult because Netgain does not publish performance details for their motors at high voltages and current. (I talked at length with Netgain yesterday and have asked for some specific performance information that they're going to try to get for me.)
If you would like to maintain 85mph continuously and your car is based on lithium, a Warp 11 and a Z2K may indeed be the safer bet; the components will certainly run cooler. However, your range will be disappointing, and again if your car is based on lead-acid batteries, you probably won't be driving long enough to build up enough heat to damage the lesser components anyway.
Torque and power are definitely confusing subjects, even more when you realize that the way we use these figures in conversation about cars is based on assumptions that no longer hold true when dealing with such a different animal as an EV, especially one based on a series-wound DC motor. I hope my attempts to explain haven't made the water even muddier than before.
Chris
--------------------------------------------------------------------------
After thinking about our discussion yesterday, we might want to look at using the Warp11 with the Z1K. It will be easy to upgrade the Z1K later, but not the Warp 11. The Warp 11 may give better durability, run cooler, and gives more efficiency per amp. Granted it weighs more which might offset those benefits, but 70lbs extra doesn't seem like a lot in a 3000lb car. Looking at the torque curves makes it look like it gets about 10% more torque per amp from the battery. Thermodynamically, this makes sense as it produces less heat and must be converting the energy into mechanical power.
If the torque per amp is 10% more than the Warp 9, and the cost of the upgrade is much less than 10% of the total project, then it makes sense to go with the Warp 11. Of course I am just guessing about the math at this point. You would think Netgain would have this all over their website comparing these motors price points, etc.
Rob
--------------------------------------------------------------------------
I seem to remember that the 11" is not very efficient at high current due to large end turn ratio, so I would be leaning toward the 9". But the Boxter is a tank at 3000# and so will be a challenge. Much will depend on the voltage and power capability of the batteries.
You may be able to hit six seconds with a Z1K and a high voltage pack, but in your case I would use the Z2K-HV or -EHV depending on your pack voltage.
I don't know the wait time, Mandi can get back to you on that.
--
I hope this helps,
-Otmar
--------------------------------------------------------------------------
Just wanted to clarify about the motor efficiency comparison. The short answer is, yes the Warp 11 is more efficient than the Warp 9, enough so that it may be justification enough to go with it, especially if your driving habits don't involve a lot of in-town driving where the motor's higher static and dynamic masses will impact efficiency in ways not accounted for in the official specs.
The *reasons* why the Warp 11 is more efficient are a bit different though. In fact, in most cases a larger motor is *less* efficient than a small one, for a few reasons. First and foremost, a larger motor has a larger brush contact area, stiffer brush springs and a larger commutator diameter. This creates a higher continuous friction loss, and the difference is easy to feel. The motor is physically harder to turn by hand. It also has a larger fan which moves more air; this creates a higher windage loss at high RPMs.
Second, while it is true that a larger motor produces more torque per amp, there's no free lunch, and "torque" and "amps" are only components of total power, not the whole picture. If there were nothing else to the equation, a massive dishwasher-sized motor making vastly more torque per amp would somehow magically produce more shaft power on the output than the electrical power you put into it. The reality is that there is a tradeoff -- "back-EMF" (voltage drop across the motor) rises with RPM faster in a larger motor. To cause the motor to draw a given current at a given shaft speed, the controller has to supply proportionately more voltage to the larger motor. Thus, the power input (volts x amps) grows to match the power output (ft-lbs x rpm). Greater torque from the larger motor has zero effect on efficiency.
Third, while a larger motor may run cooler at a given power throughput, this is in part because it has a larger surface area (internally and externally) from which to dissipate heat. Though *temperature* may not climb as high (and this is good in terms of avoiding overheat and insulation failure), total heat dissipation may be as high or higher in a larger motor. "Heat" is a measure of thermal power, "temperature" is only one component of heat.
The Warp11 is indeed more efficient than the Warp9, because the 11's design was more thoroughly dictated by Netgain with fewer concessions to any existing Warfield design. It's simply a better-designed motor for EV use, though the difference is not earth-shattering. The Warp 9 is primarily an industrial motor, with a few changes to make it physically compatible with the 9" Advanced DC motor, and a few other changes to make it better able to handle an automotive operating environment. For what it's worth, Netgain has been working on producing a variant of the Warp 9 that incorporates some true EV-oriented modifications, and they actually have a prototype now but it will not be available on the market for a few months at least. For those who aren't racing, we usually recommend the Warp 9 to save money on capacity that's not needed, and because in heavy stop-and-go driving the extra weight may more than cancel the 11's inherent efficiency advantage in steady-state operation.
Be aware that you're not just adding 70lbs to the car. Some of this is static mass, but some of it is spinning mass, which implies a much higher impact because you need to not only accelerate it forward, you also have to get it spinning; this is why we usually lighten flywheels when we're doing a conversion. All the energy required to spin up the heavier armature is lost as heat during braking and upshifting.
I've had several conversations with Netgain about documentation and making information available to customers, or at least to their vendors. They realize they have a problem, and are working to address it.
Again, I would be totally happy to install a Warp11 in the Boxster, and leave enough room in the design for a future Z2K upgrade. I'd be thrilled to show off what we're doing, and I think it would make a very impressive statement about the sort of conversion work we can do. As an added advantage to us, Netgain will be able to ship an 11 to us a month sooner than a 9", due to their production schedule (9s won't be available until October). I just want to make sure you're making this decision on the basis of accurate information and realistic expectations.
Chris
--------------------------------------------------------------------------
I really don't know what to do on this one. So, we should probably go with the most experienced opinion in the group, which ain't mine. Otmar seems to favor the Warp 9 and maybe going with a Z2K. But in his reply, he acts like he doesn't know the max voltage on these motors is 172-192V. Why else would he offer up the Z2K EHV?
Did you guys have any pull in getting the controller faster? Aaron said he thought Nov was the delivery date, but that email says 6mos!
Rob
--------------------------------------------------------------------------
Be aware that battery voltage does not equal motor voltage, and the battery voltage limit does not need to be influenced by the motor voltage limit, with the exception that it cannot be higher.
So, it would be perfectly reasonable to set a 175V limit on motor voltage for example, and run the controller from a 200V or 300V, or even 350V pack. The additional voltage gives the designer flexibility in adding more energy capacity, by simply making the string longer. That flexibility isn't as important with a lithium pack where the cells are smaller and module design is more flexible, but it still can be helpful.
Also, be aware that Otmar is a racer and a performance junkie -- like I am, but with a greater tendency to let it influence his recommendations. :o) He's right; you may or may not be able to do 0-60 in six seconds. But it will be close, 0-30 is going to be awesome, and the rest of the way up isn't going to be too bad.
One thing is for certain; Zillas are scarce and will continue to be for an unforeseeable amount of time. This means they maintain their resale value really well, should you choose to buy a Z1K now and upgrade in the future.
Chris
--------------------------------------------------------------------------
I thought we had all this worked out, but it keeps grating me to do more research... Looks like we will be going with a Z2K HV, and it will be available closer to schedule. But the motor decision is still difficult.
Based on everything you are telling me, and everything I have read, I have made some rough conclusions (possibly wrong):
The Warp 11 will be much easier to damage the transmission and clutch, but give drag racing performance off the line. It will then lose power and acceleration at freeway speeds. It will use more battery to maintain higher speeds if higher rpms are required, thus less range. But could gear shifting overcome this?
The Warp 9 will be less hard on the tranny and still go very fast off the line. It will also accelerate faster at higher rpms and possibly have the fastest top speed and longer range.
I guess if I had to choose priorities, I would like to not destroy the car, require expensive upgrades, or easily lose control and wreck the car and/or myself. I would prefer to accelerate faster from 30 to 70, than from 0 -30. I would like the car to be more efficient at 70 mph if one motor is superior to the other at 70mph constant speed.
I am thinking the Warp 9 will be the better choice, especially with Otmars comments. I did see some other forum comments about the Warp 11 using more amps compared to the Warp9 at highway speeds. If that is true, we should probably use the Warp9. I find it troubling how little commentary there is out there about this application and what works best. In fact on the evalbum pages, there are few finished cars using either a single Warp9 or Warp11 with a Zilla which quote great performance, typically using lead batteries. I see several using two 9s with Z2k, but they are trying to compete at the 1/4 mile.
Best wishes,
Rob
--------------------------------------------------------------------------
All series-wound DC electric drive systems (motor + controller) exhibit the characteristic of torque output that remains relatively constant until a certain RPM, after which torque decreases with increasing speed. That RPM "knee in the graph" is partly dependent on the motor, but mostly dependent on battery voltage. Both the 9 and the 11, and any other series DC motor behaves this way. The numbers are obviously different, but the curves are the same shape. All electric drive systems will exhibit better performance at low speeds, because below the RPM threshold the controller will be able to deliver maximum motor current.
Under 2000A, the Warp9 still poses a threat to your driveline, though not as significant as the Warp11.
The Warp9 will not necessarily provide better performance at high speed, unless you have insufficient battery voltage to continue driving the larger motor at the same current. Provided your battery voltage is high enough (200V should be), the opposite will be true.
You don't need to worry much about "damaging" your clutch; the main concern with the clutch is the nuisance of realizing that it's slipping, and needing to dismantle everything and upgrade it. This is why we specify an upgraded clutch at 1000A, and a racing clutch at 2000A.
Neither motor puts you at an unusual risk of losing control of the car, over and above the risk inherent with any powerful sports car. With 2000 motor amps either motor will provide enough torque to break traction. This is not special or unique to electric motors and as with any high performance rear-wheel-drive car, care must be taken not to apply too much power around corners without the skill to execute a controlled drift. Due to the inertia of the armature this will "feel" a bit different in an electric and will require some getting used to. Remember, you have a finite amount of traction available, and the force vectors for propulsion/braking and cornering add together to approach that limit. For example if you're coasting around a corner at the very edge of your tires' traction, any throttle at all will break the rear end loose, and vice versa.
Here's my take on the situation: In an ideal world my recommendation of 1000A and a Warp9 stands; though it may not quite deliver 0-60 in 6 seconds it certainly would come very close and would provide very entertaining performance on the street and highway. We have the problem of delivery date for the controller which has caught us by surprise, and so you've decided to pay the extra for the Z2K. You will certainly get what you're paying for, and this upgrade alone is going to give you incredible performance regardless of which motor you choose. Again, the power of the car is determined far more by the controller and batteries than the motor.
So between the Warp11 and the Warp9, the 11 will give you a modest amount of additional torque (which you don't need), and will give you higher continuous horsepower (which might be helpful if you like to cruise at 85mph). Netgain's dyno test data shows the Warp11 to be a little more efficient in steady-state operation, though they have not been able to test it across its entire performance envelope and you have seen reports from others that seem to contradict their data.
If it were me, I'd get the Warp11 (we'll note I bought a Warp13 for my truck project).
On the other hand if we're talking about a sane customer not interested in racing, I'd get the Warp9 and save some money. I think it's a given we'll be force-cooling it with a blower, so it will have a little higher continuous horsepower than stock anyway.
(For what it's worth, we wouldn't have to force cool the 11, so a little less complexity, noise and energy consumption there.)
Either way, with the Z2K, the car is going to rock. Perhaps a little too much for its own good, but we do have the ability to detune the controller's output, to a "normal driving" profile that you can change to a "pull out the stops" profile at the flip of a switch.
--
Chris
--------------------------------------------------------------------------
The difference in efficiency (and therefore range) with mixed driving will not be significant. It's hard to tell which would be better; both motors have pros and cons that would mostly cancel each other out. I believe the difference will not be sufficient to consider this an important factor.
Dual motors is a setup that's used by racers to improve horsepower. Though it seems to be something that would appeal to you, I've been hesitant to bring it up for a few reasons. Mainly, you're venturing completely into all-out electric supercar territory here, with more torque from two 8" motors than a single 11". You have the complexities of the mounting arrangement (John Wayland spent years and several revised designs trying to work out the vibration problems in his dual-motor setup, and eventually had the two motors fused together into one unit by custom motor builder Jim Husted.)
Second, to take advantage of the extra power, you'd need to raise your pack voltage, to at least 250-300V. You'd also need the series/parallel switching option on the Zilla, and the extra contactors and traction wiring that this implies.
As Aaron mentioned, this is usually the sort of setup that gets connected directly to the differential without the use of a transmission, simply because of the problems with putting that much torque through a transmission, and the absence of even high end multi-disk racing clutches that will operate without slipping. Unfortunately in your situation I think that all Boxster transmissions are of the traditional VW layout; the differential is integrated and cannot be used separately. Converting to a solid axle rear for direct drive would be an extreme undertaking, and would have effects on handling. Otmar dealt with the situation by retaining his transmission despite the use of two 8" motors, and his solution is to swap between 2 transmissions. One gets rebuilt while the other is in use.
On the other hand, dual motors would indeed get you higher horsepower at the top end. Essentially, you provide more output voltage than a single motor can stand by itself, but the voltage is split between the two motors because they're wired in series. As you speed up, the torque remains flat until a certain point, and it starts to drop off. When the Zilla calculates that it's the optimum time to do so, it cuts power for a fraction of a second, rearranges the motors electrically into a parallel configuration, and then each motor is fed full pack voltage. At high speed the motor can theoretically tolerate a higher voltage than it's rated for, and so suddenly you get a burst of additional torque. Sometimes a motor will flash over when treated like this; it's pretty unpredictable and is a risk inherent in the racing scenario for which this approach was designed. The flashover risk can be mitigated by variable brush timing, though this implies custom work on the motor (until Netgain releases their new Warp9 with built-in variable timing) and a control system for which no standard off-the-shelf products yet exist.
Another attempt at a summary:
Get the 9, save some money, have a lighter car that will handle a little better on corners due to a somewhat lighter rear end. Enjoy potentially better efficiency while accelerating, for in-town driving. Deal with some minor blower noise (which will be most obvious at a stop, when the air conditioning fans aren't running).
Or, if the extra cost isn't a showstopper, get the 11, have a bit more power and a motor that will run cooler and (according to Netgain's data) more efficiently when operated continuously on the highway. With no forced cooling necessary, the install will be simpler. Deal with some added weight.
Chris
--------------------------------------------------------------------------
Here are a couple of good web sites on K2 and A123
http://www.zeva.com.au/tech/K2/
http://zeva.com.au/A123/
I corresponded with Ian Hooper (the author of these reviews) last year on the subject of his testing, and his findings were definitely influential in our decision to go with K2 as our lithium supplier. We've built what I think is a great and promising relationship with them so far, and are in the process of negotiating some arrangements to move more volume and lower our prices in the future. Having visited their headquarters in Nevada and having seen the verification they perform on every cell during module assembly, seeing their heated and chilled enclosures for long term temperature-controlled testing, and knowing that they're actually licensed to legally sell LiFePO4 cells in the US (A123 is not licensed, and is currently being sued by Phostech and the University of Texas); I feel confident we've made the right decision.
Chris
Subscribe to:
Posts (Atom)
