Supercapacitors act like a cross between a battery and a capacitor. They last a long time and the voltage across one is proportional to the amount of charge contained, just like a capacitor. But it stores energy via an electrochemical double layer. Its volumetric efficiency compared to Lithium-ion batteries is only 2% to 5% as much. The supercapacitor is used for energy storage undergoing frequent charge and discharge cycles at high current and short duration. It assists a fuel cell by handling starting surges and energy storage during dynamic braking. As long as the total energy isn't too great. Otherwise the energy must be resistively dissipated, as with a diesel-electric.