Abstract

A large number of rechargeable unit cells connected in series forming a combination battery are divided into groups of cells, each group including several cells, preferably, three to five cells. Voltage unevenness among the cells in each group is eliminated by operation of a voltage balancer device connected to each group of cells. The voltage balancer device includes a circuit for equally dividing a terminal voltage of the group of cells and obtaining an average voltage among the cells. The average voltage is compared with an individual cell voltage, and the cells having a voltage higher than the average voltage are discharged so that all cell voltages in the group become equal. A plurality of the battery modules each including the group of cells and the voltage balancer device are connected in series to form a single combination battery, and voltage unevenness among the battery modules are controlled by another voltage control device. The voltage unevenness among cells in a group may be eliminated by charging a capacitor from cells having a higher voltage and charging cells having a lower voltage from the charged capacitor. Thus, the voltage unevenness among the combination battery cells is eliminated by a simple and inexpensive device.