PURPOSE: To charge the respective batteries of an automatic conveyer whose power source is composed of a plurality of batteries always full not to charge the batteries until their output voltages decrease below a predetermined value and avoid the degradation of the capacities of the batteries by a method wherein a plurality of changeover switches are provided in addition to a plurality of the batteries.

CONSTITUTION: Ni-Cd batteries A and B are provided in an automatic conveyer. For instance, the battery A supplies power to a load 11 first to operate it. If the automatic conveyer stops at each station during the operation period, the battery B is charged through a coupler 12 during the staying time of the conveyer. A charging status is monitored in accordance with the total charging time and charging is discontinued before the battery terminal voltage reaches an overcharge detection level. If a voltage applied to the load 11 reaches an overdischarge detection level, switches 13 and 14 which are interlocked with each other are changed over to connect the battery A to the coupler 12 and the battery B to the load 11. Thus, the load 11 is operated by the battery B and the battery A is charged.