After the case was opened, and each cell’s individual terminals were accessible, it was found that one of the six cells had been forced into a reverse voltage condition. While the other five were > 4V, or nearly full, cell #3 was not only discharged, but actually damaged beyond repair and showed a negative terminal voltage. How does this happen? Over time, when cells in a pack are repeatedly cycled without balance, mismatch can grow until a cell is destroyed. One cause of imbalance is non-symmetrical loads. For example, the battery management circuit itself could be drawing more current from one cell than others as part of the measurement circuit. If one or more cells are closer to a heat source then those cells can also be subjected to greater stress over a period of time. Some imbalance exists just from the time the cells are manufactured, although this is relatively small initially when cells are new. The purpose of cell balancing is to make sure that the small differences that cells may have initially do not increase to the point that the pack does not operate properly.