Patent ID: 7675265

Claim:
A battery charger adapted to supply current to a removable battery pack comprising battery cells and a charging terminal, comprising: a first terminal for coupling to an external power source, a second terminal for coupling to the charging terminal of the battery pack, a power source circuit coupled to the first terminal and the second terminal, the power source circuit transforming voltage input from the external power source, and supplying charging current to the battery cells, wherein the power source circuit includes a transformer which comprises a primary winding coupled to the first terminal, a first secondary winding coupled to the second terminal and a second secondary winding which is not coupled to the second terminal, wherein the voltage output from the second secondary winding has a linear relationship with the voltage of the primary winding, a voltage detector coupled to the second secondary winding, the voltage detector detecting the voltage output from the second secondary winding in order to determine the voltage input from the external power source to the power source circuit, and a processor coupled to the voltage detector and the power source circuit, the processor (i) determining the external power source voltage based upon the voltage output from the second secondary winding detected by the voltage detector, and (ii) determining a first allowable charging-current value, which permits charging while suppressing an increase in the temperature of power source circuit, based at least upon the determined external power source voltage.