TheMK wrote: ↑
Thu Jun 11, 2020 7:24 pm
My car has 50,500 miles on the odometer, and is a July 2014 build. I've done DC Fast Charge not too much, maybe 10 or so times, and never to 100%. My car is subject to both cold (below freezing) and hot (90-95 degree days) in the course of the year, but my AC compressor is strong still, so the heat should be managed.
Unfortunately, yours is not an uncommon story for 60 Ah i3 owners. The Idaho National Laboratories bought 4 new i3 BEV's and using their advanced testing equipment, measured the usable capacities of their battery packs.
Three lost 7.5% of their usable capacity after only 14 months and 12k miles. The fourth i3 lost 3.5% of its usable capacity after only 3 months and 4k miles but was either not driven for 14 months and 12k miles or the results weren't reported. That's pretty disappointing and so consistent that these probably weren't outliers.
Our 2014 i3 has been driven only 10k miles in 5.5 years. Living in Honolulu, our i3 has never experienced temperature extremes. The temperature where it has been parked 99% of its life in an apartment parking garage varies between 70º F and 78º F.
It has been DC fast charged only once from a low charge level to 80% and 3 other times for ~10 minutes each time from a low charge level. It is AC charged almost always at only 16 A. I almost always charge it to only an indicated 80% - 90% rarely charging it full and leaving it fully charged for more than a few hours. I did allow it to remain fully-charged for 3 weeks to determine whether unequal cell charge levels might be responsible for its capacity loss, but this made no difference. The battery pack has never remained at a low charge level. I occasionally allow the charge level to drop very low followed by a full charge hoping that the battery pack capacity might be calibrated to a larger value, but this has never happened.
So its battery pack has had an ideal life according to best practices. Yet it has lost more than 15% of its actual usable capacity which I can estimate by driving the same 100-mile route. This is consistent with its current 15.4 kWh Batt. Kapa. max value. What I don't know is whether the cells have actually degraded that much or whether the battery management system (BMS) has decreased the usable capacity to minimize the risk of the cells actually degrading more than the 30% warranty limit.
The fact that older versions of BMW's ISTA+ software can increase the usable capacity makes me think that the (BMS) is at least partially to blame for the capacity loss of many 60 Ah battery packs. However, this doesn't explain why some 60 Ah i3 owners claim little loss of capacity over many miles driven. Maybe there were a lot of weak 60 Ah cells installed in 60 Ah battery packs with most lucky 60 Ah i3 owners not having any of these weak cells in their battery packs. With an i3 battery pack being a single string of 96 cells connected in series, the entire pack is no stronger than its weakest cell.