A capacitor system suitable for charging and discharging of a vehicle such as discharging due to starting of an engine, charging by an alternator, and recharging upon braking. The capacitor system can be a vehicle power source having several connected cells in which ion conductive materials are arranged between a pair of electrode structures with large surface area materials and an electric double layer is formed between the large surface area materials in the electrode structure and an electrolyte of the ion conductive materials. Charging and discharging are controlled for state of charge of the capacitor unit with several connected modules to be within a predetermined range. A minimum charge within the predetermined range is sufficient for the engine to start while a maximum charge within the predetermined range is within a rated voltage of the capacitor unit with several connected modules.