The battery equalizer is used for the batteries which are connected in series to keep the voltage of batteries are equal when the batteries are charging or discharging. When the batteries work in series connection, the batteries voltage maybe will be not same due to the difference of each cell chemical composition and temperature. And each battery’s selfdischarge rate also different. So even when the batteries not work, their voltage of series batteries will be also different. These differences will cause battery lost balance, this means one battery is overloaded and the other is insufficiently charged. The voltage difference will be increased with the battery repeated charge-discharge process. This will result in premature failure of the batteries.