PURPOSE: To enhance reliability and prevent the malfunction of a car battery charging system by detecting a voltage between electrodes set specifically as required.

CONSTITUTION: A fourth electrode 25a on an unmanned carrier 20 contacts a first electrode 19a, and a fifth electrode 25b on the carrier 20 contacts a second electrode 19b. Further, a sixth electrode 25c on the carrier 20 contacts a third electrode 19c to stop the carrier 20. Voltage detection circuits 14, 15 on a charger 10 detect the voltage on a battery 22 through the second electrode 19b, the fifth electrode 25b, resistor 24, the sixth electrode 25c, and the third electrode 19c, and outputs it to a control circuit 16, which charges the battery 22. When charging is completed, the carrier 20 departs from the electrodes 19a-19c, and the voltage detection circuits 14, 15 stop detecting the voltage of the battery 22, and a switching and transforming-rectifying circuit 12a is turned off. Thus, this system is free from problems such as a conventional reed switch chattering during car battery charging, completely preventing malfunction during car battery charging.