The self-discharge of the battery refers to the phenomenon that the capacity of the battery is reduced during storage. Battery capacity loss due to self-discharge when battery is open. Self-discharge is usually mainly at the negative pole. Because the negative active substance is a more active spongy lead electrode. Lead autolysis will occur in the reaction with electrolyte (sulfuric acid), resulting in capacity loss. Besides, there are also a very small amount of harmful impurities in the negative electrode.(Such as iron ≤ 0.006%, etc.) . These impurities are deposited on the surface of the lead electrode so that formation of corrosion microcell with negative active substance lead. Lead is dissolved in this short-circuit battery pack, accompanied by hydrogen precipitation. So that the capacity is reduced. The self-discharge of the positive pole is due to the fact that during placement, decomposition of positive active substances . So that form lead sulfate and precipitate with oxygen. In addition, because the upper and lower ends of the electrode, the pores of the electrode and the concentration of acid on the surface of the electrode are different, the concentration difference between the inside and outside of the electrode and between the upper and lower ends of the electrode is formed.
  National standard stipulation that Self-discharge rate not exceeding 40 per cent within six months,hat is, < 0.22% / day.