To make it possible to freely set a power supply voltage by constituting a battery connecting body by connecting a plurality of batteries in series, connecting batteries sequentially to a charging power supply, and repeatedly charging over a predetermined period of time.

A relay switch 2 connects batteries 1a to a charging power supply 3 in such a manner that each battery 1a constituting the battery connecting body 1 can be sequentially connected to a charging power supply 3 and has a two-pole double throw contacts 2a and 2b forming a charging circuit 4 and a relay coil 2c for turning contacts 2a and 2b to ON-OFF. Relay coils 2c are built in parallel in a relay switching time oscillation circuit 7. The relay switching control circuit 6 sets a charging time as well as a halt time during relay switching and also functions as a reset circuit when turning on the power supply, controls power application to a relay coil 2c by a signal from a relay switching time oscillation circuit 7, and switches and controls each relay switch 2.