Patent attributes
In order for a backup battery to prolong a backup time period for a mobile instrument, a control signal Sc1 becomes either low or high in accordance with a determination that a power supply voltage Vm of a main battery is either lower or higher than a prescribed level V1 requiring backup from the backup battery. A control signal Sc2 also becomes either low or high in accordance with a determination that a backup voltage Vb of the backup battery is either higher or lower than a power supply voltage Vm supplied by a main battery. When both of the control signals Sc1 and Sc2 are in the high level, a control switch is turned ON so as to stop a current from flowing from the backup battery to the main battery. The control switch is otherwise turned OFF.
