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.