Patent ID: 7602082

Claim:
A power supply apparatus employing a backup battery configured to supply a load with a voltage in place of a main battery when a voltage of the main battery decreases to a prescribed level, said power supply apparatus comprising: a first terminal connected to the main battery; a second terminal connected to the backup battery; a diode having an anode connected to the first terminal and a cathode connected to the second terminal, wherein the diode is configured to output the voltage of the main battery to the load through the second terminal; a switch circuit configured to short the diode; a voltage monitor circuit configured to monitor and determine if the main battery voltage decreases to a prescribed level; a voltage comparison circuit configured to compare the main battery voltage with the backup battery voltage; and a circuit responsive to the voltage monitor circuit and the voltage comparison circuit for controlling the switch circuit such that: the switch circuit shorts the diode when the main battery voltage is equal to or higher than the prescribed level and higher than the backup battery voltage to connect the first terminal to the second terminal; and the switch circuit does not short the diode when the main battery voltage is equal to or higher than the prescribed level and equal to or lower than the backup battery voltage to output the main-battery voltage to the load while preventing a current from flowing from the backup battery to the main battery.