An energy storage device for storing energy for starting an internal combustion engine of a motor vehicle includes a DC-DC converter, a plurality of capacitors connected electrically to the DC-DC converter, and a housing for containing the DC-DC converter and the capacitors. The DC-DC converter converts a voltage provided by the motor vehicle's battery to a second voltage stored by the capacitors. During an engine start cycle, energy discharges from the capacitors to the starter motor of the engine, wherein the stored voltage of the capacitors provides energy to start the engine. The capacitors are recharged by the vehicle's battery. A thermally insulated barrier separates the DC-DC converter and the capacitors. The housing may be sized and shaped substantially as that of a standard motor vehicle battery, enabling the energy storage device to be installed within the motor vehicle as a substitute for one or more of the vehicle's batteries.