PURPOSE: To extend the life of a main battery by setting the capacity of the end cells of the main battery larger than other cells, quickly charging other cell by a spare battery at emergency, and supplying a power from the series power source of the end cell and the spare battery to a negative electrode.

CONSTITUTION: Connecting means I1 and T2 are usually connected to each other, and when a main battery A is charged, a spare battery B is charged through a diode D. When the battery A is laid in the discharge state, the battery B is not discharged and keeps the charged state. Since the capacity of an end cell 6 having the internal positive electrode terminal P1 of six cells of the battery A is larger than other cells 1-5, the capacity of the cell 6 is left even when the capacity is lacked as the whole cell. Thus, when the connecting means is switched to the connection of T2 and T3 at emergency, the cells 1-5 are quickly charged by the battery B, and since a power is supplied from the series power source of the cell 6 and the battery B to an external load, the capacity ratio of the battery A to B can be enhanced.