PURPOSE: To reduce the internal resistance, to charge at a high rate current, and to reduce the charging time, by providing a heater to heat a polymer battery to a battery charger, so as to raise the temperature of the battery.

CONSTITUTION: The voltage converter 1 of a battery charger is composed of an input terminal 2 connected to an external power source, and output terminals 3 and 4. The output terminal 3 is for charging current, and outputs the current to a connection 6 through a charge controller 5. The terminals of a polymer electrolyte secondary battery 7 are connected to the terminal 6 to carry out the charging, and the secondary battery 7 is heated by a heater 8 connected to the output terminal 4. The maximum charging current of the polymer electrolyte secondary battery is changed depending on the battery temperature. As a result, by regulating the battery temperature accurately, a constant current charging can be carried out, and the charging time is reduced.