Bottom balancing to the rescue

Last night I rode my battery pack half empty and today added the 25th cell that arrived yesterday. I then proceeded to drive the pack completely empty so I could bottom balance the new cell as well. I dropped off some badges my S.O. had made, located a gas station not to fill up but to check tire pressure, picked up two salads from the local market and ran out of juice before getting home. Yes, I thought the pack would have 5 Ah more left, but alas it did not. I kept pushing against the 50 volt limit I had set in the controller, that's only 2 volts per cell, but couldn't get home without pushing for real.

I was more than a little worried that I had incorrectly estimated the state of charge of the new cell and now ruined it straight away, but it was not so. It was happily resting at 3.2 volts while the rest of the pack was around 2.5 volts per cell. I didn't take very long to go from very worried to very happy about how well bottom balancing had protected my precious cells. I hadn't been able to break them even by pushing them down to 2 volts. And I tried. The bike would just stop going as the cells refused to give more than a few amps and floated back from their lower limits with no problems whatsoever. I can therefore testify that bottom balanced cells will protect each other and not let you kill their colleagues.

I then turned my attention to the newcomer and drained it down to about 2.5 volts where the older cells were waiting, connected my 2 kW charger and let it do it's thing. About 40 Ah went in as expected. For the bottom balancing I used the electric motor of the bike itself.