Generally a secondary battery is used with a hybrid drive in which an electric motor is driven by electric power from an engine-driven generator and/or storage battery. The secondary battery restricts the improvement of fuel efficiency by its low charge/discharge efficiency and requires regular replacements because of its short life, resulting in high running costs. An inventive drive system comprises a plurality of series-connected capacitor cells to form a capacitor bank as a storage device, parallel monitors each connected across each of the capacitor cells to bypass discharge current when a predetermined terminal voltage is reached, and switching converters having a constant current output characteristic to control charging current for the capacitor bank. The drive system achieves improvements in fuel efficiency and running cost.
