A device for determining the charge state of an accumulator and having a charging current supply unit that can be operated in a plurality of charging modes is provided. When the storage charge mode is activated, the accumulator is charged only up to a predefined, optimized partially charged state. If the charge state determined at the start of the storage charge mode is higher than the predefined partially charged state, then the discharging device discharges the accumulator as far as the predefined partially charged state.