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.
Sunday, February 20, 2011
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.
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