A battery charger for charging a lithium-ion battery includes a temperature control unit and a current supplying unit. The temperature control unit controls a temperature of the lithium-ion battery to fall in a predetermined range. The predetermined range is divided into a plurality of sub-divided temperature ranges. The current supplying unit supplies the lithium-ion battery with a temperature-dependent current relation to a sub-divided temperature range in which the temperature of the lithium-ion battery falls.