Li-ion battery, as a new type of energy storage equipment, has small size, large power density, high individual voltage, low rate of self-discharging and small self-resistance etc. It substitutes the traditional power battery in AGV.In practical process, due to the difference of battery cells, after a period of charge-discharge time, the upper and lower voltage limits of every unit of batteries become various, seriously affecting the system performance. The idea of upper and lower equalizer is released for this case. Then, a further research is made on the equalizer circuit of li-ion battery. The experiment result proves the correctness of design on several types of lithium battery equalizer circuit.