PURPOSE: To allow a lead-acid battery to exert its cycle life characteristics fully through improvement of the two-step constant current charging method.

CONSTITUTION: In a two-step constant current charging system in which the terminal voltage of a lead acid battery being charged is sensed and charging- over takes place from the first step to the second, the second step charging time is decided by reference to the charging time from the start of the first step till changing-over to the second step, and by the use of this means, a proper charging amount of electricity is supplied to the lead acid battery for use in the field where the discharge amount of electricity before charging is not decisively determined, and thereby a charging without excess nor shortage should be achieved. This permits accomplishment of a charge pattern with the number of times in the cyclic lifetime increased very much, compared with the constant voltage charging system used generally for charging a lead acid battery as conventional technique.