PURPOSE: To promptly correct erring connections with battery terminals of voltage different from rated voltage in a distributing system with different voltages by a method in which the oad of the distributing system is interrupted from power source and at the same time a warning is sent out.

CONSTITUTION: The 24 V terminal 2 of the three-terminal battery 1 is connected with the load 5 from an engine start-up system, and the 12 V terminal 3 is also connected with the load 6 from a lighting system, an air conditioning system, a power system, etc., for example. The 24 V distributing system is connected with the PNP transistor 7 as low voltage detection means, the relay coil 8 controlled by the transistor 7, and the warning lamp 11. Also, the 12 V distributing system is connected with the NPN transistor 13 as a high voltage detection means, the relay coil 14 controlled by the transistor 13, and the warning lamp 16.