It's a lithium iron phosphate battery, also known as LFP or LiFePO4. It's got 16 cells wired in series to make a nominal 48 volts.
The battery is considered to be fully charged when each cell reaches 3.65 volts, or 58.4 volts total. The battery is considered to be fully drained when any cell reaches 2.7 volts. Ideally they'd all go up and down evenly, because they're each supplying 1/16 the total voltage.
Cell imbalance exists because LFP cells aren't all made exact. There is little variances from one to the next. Since this was the cheapest 48v 150Ah battery I could get, the cells are probably not top quality either so they're more likely to have this issue.
Usually what happens with cell imbalance is that one or a few will reach fully charged before the rest, at which point the battery stops charging. So some of the cells are full and some are almost full. The same thing happens at the bottom. One cell reaches 2.7 volts before the rest, the battery turns off. This is managed by a BMS, battery management system, a small built-in computer that keeps the battery from destroying itself. The BMS has some capability to balance cells on it's own during charging. You leave it on the charger for a few hours after it's "done" and the BMS will bleed power off of the more charged cells to get them away from 3.65 volts, so the charger will start charging again with the hopes that the less charged cells will charge up slowly. Sometimes this works and sometimes it doesn't. There are BMS that have active balancing where it can charge each cell individually, but these batteries don't have that. It might be an upgrade I do later, though.
The capacity loss from not being fully charged or fully empty is minimal, but the wear and tear difference from one cell to another will become a problem later as the battery gets old. The imbalance will get worse over time.
I can fix this manually by taking the top off of the battery and charging each cell by hand with a bench power supply. That's probably gonna take more than 50 hours of my time though considering I have three of these things, or about 1/4 to 1/3 the average electric cars worth.
I bought them on Amazon, so I have 30 days to figure this out. After that I'm almost certainly stuck with them.
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