To provide a method and a system for charging the battery of an electric tool.

The battery charging system comprises a first battery, having the chemistry of lithium base, a second battery having the chemistry of lithium base and a nominal voltage different from a first nominal voltage, and a battery charger which can operate to charge the first battery and the second battery. In an electrical combination, comprising a first battery having the chemistry of lithium base, a second battery having the chemistry of lithium base and a second nominal voltage different from a first nominal voltage, and a battery charger which can operate so as to charge the first battery and the second battery, the method for charging the battery comprises a step for connecting the battery charger and the first battery electrically, a step for charging the first battery, a step for connecting the battery charger and the second battery electrically, and a step for charging the second battery.