One Cell Goes up Faster – End of Charge Cycle

If you keep a close eye on the cell voltages, you may see one of the LiFePO4 cells appear to “shoot up” or “race” higher than the BMS can compensate for by balancing. If it happens, it’s typically once the cells are over 3.4 volts each.

Once that cell that’s “racing” hits 3.65V the BMS will trigger “Cell Overvolt Protection” and turn off the charge path via the Mosfets saving the cells from damage. The absolute maximum voltage for a LiFePO4 cell is 4.2V, so it’s well within the maximum limit.

Check out this study/chart to see it graphically and read a short summary.

https://www.powerstream.com/lithium-phosphate-charge-voltage.htm

The gest of that study is that 3.4V is considered fully charged for a LiFePO4 cell, and from 3.4 to 4.2 you only gain an additional 1-2% of capacity. There will always be one cell at some point, maybe at 3.4 it starts, or maybe at 3.5 it starts, but it will almost always happen.

But, aren’t the cells mis-matched or not matched for internal resistance if they race?

No, if they all make it to 3.4Volts (fully charged) then they are considered matched for production standards and the battery has met its capacity rating. If you purchase a 100Ah battery and you get 100 amp hours from it without damaging the battery, then the cells and battery are performing as advertised.