Patent ID: 8035347

Claim:
A battery charger for charging a secondary battery using a power supply circuit which converts an AC input into a DC output, the battery charger comprising: a discrimination circuit to discriminate a constant-current charging mode and a constant-voltage charging mode, and generate a discrimination signal, the constant-current charging mode charging the secondary battery at a constant current, the constant-voltage charging mode charging the secondary battery at a constant voltage when a terminal voltage of the secondary battery of the constant voltage reaches a predetermined voltage, and the constant-voltage charging mode being shifted to an end of charging mode when a charging current is lowered to an end of charging detection current; and a controller to which the discrimination signal is supplied, the controller generating, as a current of a constant current control, a control signal to switch between a current in the constant-current charging mode and the end of charging detection current, wherein: when judged as being the constant-current charging mode from the discrimination signal, the controller sets the current in the constant-current charging mode by using the control signal, when judged as being the constant-voltage charging mode from the discrimination signal, the controller sets intermittently the end of charging detection current by using the control signal, and sets an end of charging detection period for judging the constant-current charging mode and the constant-voltage charging mode in accordance with the discrimination signal, and when the discrimination signal indicates the constant-voltage charging mode in the end of charging detection period, the controller controls to shift to the end of charging detection mode, the battery charger further comprising first and second resistors inserted in series into a charging current path, wherein: the controller performs the constant current control by using a first detection voltage generated at the first resistor when setting a current in the constant-current charging mode, and in the end of charging detection period, the controller switches to perform the constant current control by using a second detection voltage generated at a combined resistance of the first and second resistors connected in series.