How does a capacitor work? What does that have to do with electric vehicles? First, let’s look at capacitors.


Basically, A capacitor works by storing energy on one plate, then releasing that energy to another plate. This article will outline the operation of the parallel plate capacitor. The discussion is simplified for clarity, and leaves out the Calculus.

Picture two close together metal plates. The distance between the plates may vary from nanometers to centimeters. The plates may be round, square or cylindrical, or other shape.

The medium between the plates can be air, vacuum or some other material. The material between the plates is called the dielectric medium. The amount of charge or Capacitance varies with the distance between the plates, the permittivity of the medium between the plates, and the area of the plates.

Capacitance then increases with plate area and permittivity. It decreases with distance apart of the plates.

The resulting equation is: Capacitance is equal to the permittivity (dielectric) x plate area / distance between the plates.


When the battery or energy source in the capacitor circuit is switched on, charge starts to build up on the nearest capacitor plate. When the maximum capacitance is reached the plate will not accept any more charge. The charge will be maintained on the plate as long as the power supply is maintained. Once the power is switched off, the charge in the capacitor will begin to discharge its energy to the other plate.

The stored energy will continue to flow across the plates, and on down the circuit to the device needing power.

After the capacitor discharges, it will simply recharge if the switch is opened. This cycle will repeat itself hundreds, thousands or millions of times. That is the great part about capacitors. There are no moving parts. They last a long time.

While capacitors are great at storing and releasing charge on cue, they have a couple of practical limitations. First, they can not store charge for very long, and they have low energy density. They can not hold as much energy as say lithium ion storage batteries. Research is ongoing to improve both limitations.


So, now you know the basics of how does a capacitor work. How do capacitors help electric vehicles? The main use right now is for braking. Since capacitors can accept energy at a fast rate and batteries take a long time to charge, they are well suited for storing energy normally lost to heat during braking.

Take hybrid vehicles for example. As hybrids brake, their on board alternator can also be used as a generator. As the brakes are applied, the generator kicks in slowing the car and recharging the batteries and capacitors at the same time. Some hybrid taxi drivers for example have reported a lot less brake wear due to the use of regenerative braking. Pure electric vehicles like the new Nissan Leaf also have regenerative braking.

In the future, it appears possible that large capacitors called ultracapacitors or ultracaps could replace batteries altogether. Large ultracaps can be very quickly charged. However, ultracaps today will not hold energy for long, and cannot yet be relied on for long term storage like batteries. Research is ongoing.

Banks of ultracaps have found use in some city buses however. Currently in China, ultracap powered Sinautec buses can Hot Springs travel around 5 miles between charges. The buses connect to overhead power lines at bus stops. Since the bus needs to make passenger stops anyway, as they are stopped, they charge up their capacitor banks.

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