In the process of battery discharge, the positive and negative active materials are gradually transformed into PBSO4 with high resistance, and consume the sulfuric acid in the electrolyte, and the internal resistance gradually increases. Therefore, when the battery is over-discharged, especially when the current is over-discharged, it will emit A lot of heat, and the amount of sulfuric acid in the battery is very small. The sulfuric acid concentration is reduced very low during overdischarge, and the solubility of PBSO4 is greatly increased. Therefore, it is easy to form a kind of coarse and hard PBSO4 crystals on the electrode plate, that is, "irreversible sulfation". Ground weakening of the battery's charge acceptance is particularly harmful.