Patent ID: 7764043

Claim:
A battery charger comprising: a first charging circuit for controlling input power to charge an external battery which is detachably mounted to the battery charger; an internal secondary battery which can be charged by the input power; a charge/discharge control circuit for charging the internal secondary battery by the input power and controlling a discharge of the internal secondary battery; and a control circuit for controlling an operative state of the first charging circuit and the charge/discharge control circuit, wherein the battery charger serves to charge the detachably mounted external battery by means of the input power as well as by means of the internal secondary battery, wherein the battery charger is so constructed and arranged that when the input power is not in a state of being inputted to the battery charger, the control circuit supplies electric power from the internal secondary battery to the charge/discharge control circuit in an operative state, so that the charge/discharge control circuit in its operative state controls a discharging state of the internal secondary battery, and thus the internal secondary battery is discharged, so that the external battery is charged by using the discharged electric power from the internal secondary battery, and wherein the charge/discharge control circuit includes a charge switch for controlling charging of the external battery with the internal secondary battery, the charge switch being manually selectable between ON and OFF when the external battery is mounted to the battery charger.