A capacitor is an energy storage device. It can only store electrons electrostatically. This is different from an electrochemical battery which actually creates its own electrons through chemical reactions. Recharging a battery returns the products of the chemical reactions back to reactants so that the reaction can take place again. A capacitor consists of two metal plates with an insulator (dielectric) in between the plates. When a voltage source such as battery is connected to the two plates (one plate is positive and the other plate is negative), it forces electrons to the negative plate and removes them from the positive plate. To help visualize this process, one can think of a capacitor as a pipe with a flexible membrane in the middle and water on both sides of the membrane. As water (electrons) flow into one side of the pipe, the membrane flexes and pushed water (electrons) out of the other side. There is a point where the membrane will not flex anymore, this is when the capacitor is saturated and it will not hold anymore electrons. Now, if one connects a load (resistor, light bulb, etc.) to the positive and negative side of the capacitor, electrons will flow out of the negative plate and back to the positive plate. The amount of charge a capacitor can hold is known as capacitance (C) and is measured in Farads. The more surface area the plates have, the more charge a capacitor can hold. Most capacitors are in the microfarad range. The total amount of energy a capacitor can hold be represented as energy = (1/2)*(C)*(V)^2 where V is the voltage of the capacitor when it is saturated.
Supercapacitors have plates made of porous activated carbon which creates an enormous surface area, thus yielding a large value of C (in the several thousand Farad range). Unfortunately, the amount of energy a supercapacitor can store will probably never be equivalent to that of battery. However, due to the very low internal resistance, supercapacitors can be charged in a matter of seconds. Additionally, since there are no chemical reactions, they can be charged millions of times with no decrease in capacity. The low internal resistance also means that supercapacitors can discharge very quickly with a high current, making them useful for providing EVs with short bursts of power. For example, a short burst of power is needed for climbing hills and merging onto highways. This will reduce drain on the electric vehicle’s main battery and increase its range. The supercapacitors would be recharged from regenerative breaking. This process would be analogous to the a gasoline vehicle’s accelerator pump. Supercapacitors could never be the soul power source of an electric vehicle due to their low energy density, they can only supplement the battery.