Types of Polarised Capacitors
For a basic capacitor explanation, you have two plates separated by an insulator. The plates store energy, sort of like a battery. The big difference between batteries and capacitors is that a battery supplies a nearly constant voltage, whereas a capacitor works at all voltages up to its maximum
safe rating. We will review the basic polarized capacitor, of which there are several kinds.
Basically, when a DC voltage source is applied to a capacitor there is an initial surge of current. When the voltage across the terminals of the capacitor is equal to the applied voltage, the current flow stops. When the current stops flowing from the power supply to the capacitor, the capacitor is charged up.
After the capacitor is charged up, it can be discharged like a battery. The only problem is that the capacitor will not want to hold its charge for more than a few minutes. This presents obvious problems using the capacitor as a storage device.
There are many different materials used to make capacitors. These include metals such as aluminum, tantalum, and silver, for example. Other materials include glass, paper, polycarbonate, polystyrene and Teflon, to name a few.
For electric vehicles, a capacitor explanation needs to include the electric double layer supercapacitor, or ultracapacitor. These capacitors offer the higher energy densities required for the EV.
Like the storage battery, working capacitors have been around for about 150 years. Small scale capacitors are great for electronics applications but have a hard time storing enough energy long enough to drive a car very far. Over the past several years a company in Texas, EEStor, has repeatedly claimed to have perfected an ultracapacitor with a long resident storage time. So far, this and other claims have been unfounded.
The chart below shows that the general electrostatic capacitor (bottom of chart) holds about as much energy per weight as a nickel metal hydride (NiMH) battery. The Toyota Prius has a NiMH battery pack for example. To power an electric vehicle, you need an ultracapacitor. Chart 2 shows EEStor claimed specs. The specs show an ultracapacitor with a very quick charge-up time, long storage time, with half the weight of current Li-Ion battery packs. Too good to be true? News sources seem to think so.
However, dedicated research on the ultracapacitor is ongoing at MIT. In China, city buses are now powered by the ultracapacitor for short distances. This works out well since the big caps can be quickly recharged at bus stops along the way.