Regarding using four individual 12v LFP batteries to make a 48v battery came up last night, so I want to expand on it a bit today.
First a little back ground on LFPs... they are made up of indiviadual cells that run about 3.2v each. As a battery nears full, some of those cells will pop up to 3.6v. The BMS will try to make that one cell match the others that are under it's control... but if that one cell gets above 3.6v, the BMS will stop allowing the battery to charge.
My mantra is always "it's not THAT it works... what's important is how it works when something is wrong".
So imagine we've got 4 groups of 4 cells. Each group of 4 cells is a 12v battery. If any cell 1-4, 5-8, 9-12 or 13-16 gets to 3.6v the entire bank of batteries will stop charging because one BMS will stop charging while it whittles down the excess voltage. That excess voltage can only be distributed to other cells in the same group of 4. So now imagine that cell 3 gets too high.... everything stops, and cell 3's voltage is distributed into cells 1,2 and 4. 10 minutes later, charging resumes but now cell 11 is over voltage. So the BMS for that group of 4 stops the whole string of batteries from charging and distirbutes the voltage from cell 11 to 9,10 or 12.
Further, when your four 12v batteries are add up to 55.2v in total that doens't mean that battery 1 isn't at a significantly different voltage than batteries 2,3,4. We'd like to think that each battery (each group of 4cells) is at 13.8v, but it could be that one of them is at 14.2v and the others are 13.6v. None of the four 12v batteries has any way to balance themselves against the other 3.
Now travel back to the beginning with me to the day we bought a battery. Today we buy a 48v LFP battery. It has one BMS that monitors 16 cells. When one of those cells hits 3.6v, that extra voltage isn't just shared among the 4 closest cells... that excess voltage is distributed to 15 other cells. The result is that when your battery is at 55.2v, each of the 16 cells is at exactly the same voltage.
Hope this makes sense. Just because you can use four 12v batteries to make a 48v system doesn't mean you should. You are setting yourself up for balancing problems in the future.
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