Patent ID: 7808209

Claim:
An electronic device, supplied with a battery voltage from a battery, comprising: a first power supply unit that converts the battery voltage to a first constant voltage; a second power supply unit that converts the battery voltage to a second constant voltage, which is higher than the first constant voltage; a constant current load unit that outputs a constant current; a load switching unit that connects the first power supply unit to the constant current load unit if the battery voltage is higher than a predetermined voltage, and connects the second power supply unit to the constant current load unit if the battery voltage is equal or less than the predetermined voltage; and a remaining charge calculator that calculates a remaining charge of the battery based on a first voltage, a second voltage, and a third voltage, wherein the constant current load unit outputs the constant current to the load switching unit, and wherein the first voltage is the battery voltage detected if the constant current is not outputted to the load switching unit, the second voltage is a load voltage supplied to the constant current load unit if the constant current is not outputted to the load switching unit, and the third voltage is the battery voltage detected if the constant current is outputted to the load switching unit.