An energy storage system for a vehicle includes an ultracapacitor that is electrically coupled in parallel with a battery through a diode connected in series with the battery. The ultracapacitor delivers the energy required for all high current surges that occur during engine start, acceleration, and regeneration. The battery is used only to assist with longer duration, high energy loads, such as accessory loads when an engine of the vehicle is not running. In other words, the battery conducts only during longer and less-frequent pulses and, therefore, does not have to withstand high-power pulses.