It seems that my KP-E charger had started to malfunction and didn’t charge my batteries completely. After the faulty charger had completed, the voltage was showing about 81.5V in Cycle Analyst, instead of the 82.1V that it used to, when the charger was still working properly (the CA has always shown lower values than my multimeter and 82.1V in CA while bike is powered on equals about 84V in my multimeter when it’s powered down). I had also measured the cells and they all showed steady voltages, so I didn’t suspect it would be because of a bad cell, jumping too high earlier than the others, cutting off the charge. Actually the charger was my suspect #1, since earlier in the summer it had blown some blue smoke with a loud “pop” once when I had connected power to it. But it had started to charge – and visually it was doing it’s job. So I kept using it. But I should have been more suspicious since the voltages weren’t what they should have been.
And since the pack wasn’t charging full anymore, one day when I was driving back home from work, the voltages started dropping really fast and the pack was showing near empty. I had barely driven 25Ah and it should have still had plenty of power (batteries are rated for 40Ah). Luckily there was just enough juice to get home, but when I got there all the cells were just below 3 volts (near empty), except for one that had dropped to about 2 volts. I immediately charged this cell higher with my Ultramat RC charger (in the picture) and it bounced up back real nice. I balance charged it couple times, matching the other cells at about 3 volts.
Luckily Jarkko was able to provide me with a new charger, as his own KP-E was laying around unused. And it seems his charger did the trick and charged the pack full. So I owe him my gratitude for getting up and running so fast again. After the first charge with Jarkko’s charger, the pack showed steady 82.1V again, even after couple days of being idle.