Batteries usually have to complete two tasks, the first is as soon as possible to restore the battery rated capacity, the other is to use small current charging battery, supplemented by self discharge and loss of energy, in order to maintain battery rated capacity. During the charging process, the lead sulfate on the negative plate of the lead acid battery gradually precipitated lead, and the lead sulfate on the positive plate gradually generated two lead oxide. When lead sulfate and two lead oxide are completely produced on the positive and negative plates, the battery begins to overcharge and produce hydrogen and oxygen. In this way, water in the electrolyte will gradually decrease in an unsealed battery. In sealed lead-acid batteries, hydrogen and oxygen can recombine into water when the medium charge rate is adopted. The time at which the charge begins is related to the rate of charge. When the charging rate is greater than C / 5, the battery capacity has returned to 80% of the rated capacity, that is, the charging reaction has occurred. Only charging rate is less than C / 100, to make the battery restored to 100% in the capacity, there has been a charging reaction. In order to restore the battery capacity to 100%, a certain overcharge reaction must be allowed. Overcharge reaction occurred after the cell voltage increases rapidly and reaches a certain value, the rising rate decreases, and the voltage of the battery began to slow down. As a result, the best way to maintain the capacity of a battery is to add a constant voltage at the end of the battery pack when the battery is fully powered. The floating voltage, current charging should be able to replenish the battery for self discharge and lose energy. The floating voltage is not too high, so as to avoid serious overcharge and shorten battery life. Appropriate float voltage, battery life up to 10 years. Practice has proved that the floating charge voltage of floating charge voltage and provides the actual difference of 5%, maintenance free battery life will be shortened by half. The voltage of lead-acid battery has a negative temperature coefficient, the single value is 4mV / C. When the environment temperature is 25 DEG C is ideal ordinary (no temperature compensation) electric vehicle charger, when the ambient temperature is reduced to 0 DEG C when the battery is not fully charged, when the ambient temperature rises to 50 degrees, the battery will due to overcharge and shorten the life of serious. Therefore, in order to make sure that the battery is just enough electricity in a wide temperature range, the switching voltages of the electric vehicle charger must change with the temperature coefficient of the battery voltage.