I am struggling with my first ever DIY-from-cells battery. It's a 4S LiFePO4 bank using 280Ah cells from SolarSupplyHouse. After being top balanced according to the protocol suggested on this forum, the cells won't stay even close to balanced for more than a few days of light usage. I use this cell in a camper and the goal is to be able to keep my tiny 12V fridge and CPAP on for a long period of clouds, not to support crazy high loads for heating or AC or something. So a normal day sees me pull about 300-350Wh out of the bank and then put it back in the next morning with solar. The most I've ever done in a single day was 750Wh (according to the Victron app) after a couple of days of clouds. Yet by the 5-6th day, the Overkill BMS will disconnect the battery from charging because one cell will go above 3.650V even while the bank voltage is under 13.500V. Most of the time, at least two of the cells are still under 3.400V when the overvoltage protection kicks in.
I labeled the 4 cells A through D so I could track what cell is doing what as I tested things. Looking at the cell voltages through the Overkill BMS, Cell A will always spike during charging first. During the 2 full discharges I did just for testing, Cell A was also the first to hit the low voltage cutoff. To me, that sounded like Cell A must have a smaller capacity than the other cells if it hit both the top and bottom limits first. In general, Cell C is right on A's heels, so I assumed A and C must be very close in capacity. Cells B and D seemed to stay pretty near each other as well but, of course, since they weren't hitting the steep low/high parts of the curve, it was harder to tell for sure.
In frustration, I bought one of the cheap battery capacity testers Will has shown in his videos. I charged each cell up to 3.600V by itself, then discharged it via the capacity tester. I was really shocked at how close the cells are! The biggest difference is between Cells A and B and that's only 2%!
How close do cells have to be capacity matched? Could just the 2% difference I found let the battery bank drift out of balance in under a week?
I labeled the 4 cells A through D so I could track what cell is doing what as I tested things. Looking at the cell voltages through the Overkill BMS, Cell A will always spike during charging first. During the 2 full discharges I did just for testing, Cell A was also the first to hit the low voltage cutoff. To me, that sounded like Cell A must have a smaller capacity than the other cells if it hit both the top and bottom limits first. In general, Cell C is right on A's heels, so I assumed A and C must be very close in capacity. Cells B and D seemed to stay pretty near each other as well but, of course, since they weren't hitting the steep low/high parts of the curve, it was harder to tell for sure.
In frustration, I bought one of the cheap battery capacity testers Will has shown in his videos. I charged each cell up to 3.600V by itself, then discharged it via the capacity tester. I was really shocked at how close the cells are! The biggest difference is between Cells A and B and that's only 2%!
How close do cells have to be capacity matched? Could just the 2% difference I found let the battery bank drift out of balance in under a week?