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Building a 12V LiFePO4 battery with components from a 48V battery with one bad cell

208stephen

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In another post I was attempting to repair a bad string of cells in a Discover AES 48V battery. After disassembling the battery and exploring my options it's looking like, short of radical "surgery", that the battery is toast. However, the way the battery is constructed there are 4 subunits of cells, each containing 4 parallel strings of 15 26650, 3.2V, 3.8ah cells (see attached photo). One of these subunits has a bad string but the other 3 are fine and appear to be fully charged. Rather than try to recycle them I'm curious if they could be reused and configured with a BMS as 12V batteries. It would be easy enough to connect the 4 15P strings in series giving me a 12V 57ah battery.

What would I need to do to do this? Any recommendations on a quality BMS? Are there any other components that I would need to do this safely? When I took the original battery apart there were 2 circuit boards besides the BMS.
 

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so you have 45 good cells?

There use 40 of them and hold 5 in reserve. Build a 10S4S battery. There are lots of online resources for doing it with 18650. Size is the only difference with 26650.
 
Maybe I wasn't clear. The battery I took apart has 4 subunits as shown in the picture. Each subunit contains 4 15P strings connected in series for a total of 60 cells per subunit. There is one bad 15P string in just one of the subunits. The other 3 subunits each have 60 good cells and those are the ones I want to possibly use for 12V battery(s). Because of the way they're built I really want to keep the subunits intact. All the cells are spot-welded to large busbars and I definitely don't want to break the welds and re-weld them.
 
Sorry. I think you did explain it. I just didn't get it.

Okay... 3 sub-units that are 15P4S.

Just treat each subunit as a 4S 57Ah battery, put a BMS on it and then parallel the three subunits as though they were separate 12V at their main (+) and (-) terminals.

BMS rating would be based on the cell charge/discharge ratings * 15 cells * 0.8 (a little safety factor).

It also depends on your budget for a 171Ah 12.8V battery, I doubt you want to spend a whole crapton on BMS. I'm generally not a fan of DALY BMS, but there are some pretty cheap "dumb" ones:


If you go smartBMS:


Lots of option here and elsewhere.
 
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