During the working process of the battery, due to the difference in the chemical composition and temperature of the battery cells, the charging and discharging of the two batteries will be different. Even when the cells are idle, there will be an imbalance between cells connected in series due to varying degrees of self-discharge. During the charging process, due to the existence of differences, one battery will be overcharged or discharged, while the other one will be undercharged or discharged. As the charging and discharging process is repeated, this difference will gradually intensify, causing the unbalanced rehydration water of the single cell to cause inconsistent voltage and discharge of the single cell, which will eventually lead to premature failure of the battery with too much water loss in the single cell and make the whole battery The battery pack is damaged.