A set of batteries consists of three to four. Due to the manufacturing process problem, the absolute balance of each battery cannot be achieved. The average current of the ordinary charger is used to make the single battery with the small capacity first filled and overcharged. When discharging, this small battery is placed first. Finished and formed overdraw. In the long run, the vicious circle makes the whole battery appear single behind, so that the whole battery is scrapped. The three-stage charger has a small current of 500mA in the floating charging stage. Its function is to compensate for the charge and to make the battery fully charged. But it also brings two side effects: 1. After filling, the excess current is not turned off, the electric energy is converted into heat energy, the water is decomposed, and the water is dissipated; 2. The small current is charged, and the current bifurcation is large. It is more likely to cause imbalance in the battery pack.