Overcharging is the most destructive element in battery service. During overcharging, excessive current causes the oxides on the plates of the battery to ‘shed’ and precipitate to the bottom of the cell and also heat the battery, thus removing water from the electrolyte. Once removed, this material (which represents capacity) is no longer active in the battery. In addition, the loss of water from the electrolyte may expose portions of the plates and cause the exposed areas to oxidise and become inactive, thus reducing additional capacity. Sealed batteries are not immune from the same internal results when overcharged. In fact, sealed batteries are particularly sensitive to overcharging. Once moisture is removed from the battery, it cannot be replaced. Portions of the battery damaged due to overcharging are irretrievable. However, if detected early, corrective adjustments to the charging device will save the undamaged portion of the battery. Initial signs of overcharging are excessive usage of water in the battery, continuously warm batteries, or higher than normal battery voltages while under the influence of the charger.

Undercharging over a period of time also damages batteries. Insufficient charging leads to a problem called sulphation of the plates. This is a typical problem for long back duration UPS’s where the charger current capacity is less than 10% of the battery current rating. For example, while using a 100AH battery, the charger capacity should be ideally 10 amps.