PURPOSE: To operate a primary or a secondary battery effectively upto the termination voltage thereof by controlling the termination voltage of primary or secondary battery power supply to an electronic appliance while setting the set voltage for a comparison circuit individually.

CONSTITUTION: The battery voltage detecting circuit of a power supply battery 1 for an electronic appliance 9 which can operate with primary or secondary battery comprises a second negative electrode terminal 3 to be connected with the negative electrode of a secondary battery, and an electronic circuit 7 operable upon connection of the second negative electrode terminal 3 with the negative electrode. Set voltage Vref for a comparison circuit 6 is set through the electronic circuit 7 thus performing ON/OFF control of a switch 8.