BATTERY ISOLATOR

PROBLEM TO BE SOLVED: To prevent overdischaring of both a main battery and a sub battery and avoid an inconvenience between these batteries by providing a main battery monitoring circuit, a sub battery monitoring circuit, and a current controlling circuit which controls current to the sub battery based on the voltage monitored by the main battery monitoring circuit. SOLUTION: A main battery monitoring circuit 11 outputs a detection signal when the voltage of a main battery 5 becomes a preliminarily set value. A sub battery monitoring circuit 12 outputs a detection signal when the voltage of a sub battery 6 becomes a preliminarily set value and then cuts off the supply of power to a load 10. A current cut-off circuit 13 controls charging current, including the cut-off of power to the sub battery 6, based on the current running in itself and the detection signals from the monitoring circuits 11, 12 and it also stops backward current to the main battery 5. By this method, the main battery 5 is charged prior to the sub battery 6 and there is no overdischarging of the main battery 5.