Overcharge means that the charging current of the battery is greater than the acceptable current of the battery. The excess is the amount of overcharge. Overcharge is mainly a side reaction of electrolyzed water. Since the positive electrode of the battery transfers oxygen to the negative electrode, the oxygen recombination reaction occurs, and heat will be generated. The amount of overcharge is actually converted into heat to increase the battery temperature. If it is not controlled, it will cause a large amount of water loss. In severe cases, it will cause "thermal runaway" to reduce the capacity, or even deform. Generally speaking, undercharging means that when the battery is not fully charged or fully charged, it will gradually form a kind of thick and hard lead sulfate, which is almost insoluble, that is, the so-called "irreversible sulfation", which cannot be used by ordinary methods. Charged, so the capacity will quickly decay one at a time.