Battery pack may include rechargeable battery cells Battery charger may include power source circuit. Power source circuit may be connected with an external power source and battery cells. The external power source may supply power to the power source circuit and then, the power source circuit may supply charging current to battery cells. Battery charger may also include voltage detector for detecting the voltage input from the external power source to power source circuit. Battery charger may further include processor for controlling power source circuit. Processor may determine the amount of charging current supplied to battery cells based upon the external power source voltage detected by the voltage detector.