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[DV1YKL]

While building a 4s LiFePO4 pack, I noticed a potential design flaw while trying to characterize the BMS behavior. As it turns out, there seems to be a design flaw in the BMS circuit.

1. When any cell is full, the BMS stops the charger
2. While the charger is off, the BMS removes some charge from the cell with the highest voltage
3. After that voltage drops sufficiently, the BMS restarts the charger

from stackexchange

There have been documented issues with the BMS for li-ion/lipo chemistries where the HY2213 balance chip's release voltage (4.19V) is still too high for the DW01 charger chip's overcharge release voltage at 4.05V.

For the LiFePO4's case, it seems as if the manufacturer swapped the DW01 chip with the FM2112 chip for a more appropriate overcharge voltage of 3.75V and release at 3.6V, but failed to replace the HY2213 balance chip. Once one of the cells reaches 3.75V, the charger stops. But, since the balancer still activates at 4.2V and releases at 4.19V, the cell never gets balanced back down to the overcharge release voltage.

As a result, one cell gets charged to 4.2V and the rest stays unbalanced.