FIGURE. Ultra-capacitor cell construction.
Ultra-capacitors are capacitors constructed to have a large electrode surface area and a very small distance between the electrodes. Unlike conventional capacitors that use a dielectric, the ultra-capacitors use an electrolyte. It also stores electrical energy at the boundary between the electrodes and the electrolyte. Although an ultra-capacitor is an electrochemical device, no chemical reactions are involved in the storing of electrical energy. This means that the ultra-capacitor remains an electrostatic device. The design of the ultra-capacitor increases its capacitance capabilities to as much as 5000 farads.
Ultra-capacitors are used in many present-day hybrid vehicles and in some experimental fuel cell EVs because of there ability to quickly discharge high voltages and then be quickly recharged. This makes them ideal for increasing boost to electrical motors during times of acceleration or heavy loads. Ultra-capacitors are also very good at absorbing the energy from regenerative braking.