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.
No cell need be charged above 3.5 volts, ever. There is simply no worthwhile capacity beyond 3.4Volts.
ReplyDeleteAny more will not eek more life out of it. Quite the opposite. It doesn't work anything like a Lead acid battery. :(
The motto should be never over charge or discharge and never let any Cell go over 50C on electrolyte temperature.
Drill a cell bolt and fit a probe like they sell for solar heating units to measure the batteries core temperature for all conditions.
All the best.