What's the difference between Capacitors, Ultra-Capacitors and Batteries Capacitors are known to hold and release energy very quickly, unlike the slower release that batteries exhibit. If one were to bunch many (1000's of) capacitors together could they function as a battery, since each capacitor would hold and release a comparatively smaller amount of energy when compared to a battery's output at any given moment, but in total the sum of the energy stored in all the capacitors would allow it to exhibit the slower release and longer availability of stored energy of a battery?
 A: Yes, capacitors could function as a battery, but there are some technical problems.
A battery generates electricity from a chemical reaction so the voltage and power produced remains approximately constant for the lifetime of the battery. The voltage on a capacitor is proportional to the charge on it so the voltage decreases as you discharge the capacitor. This means you'll need some form of voltage regulation that you wouldn't need with a battery.
I believe that ultracapacitors currently don't store enough energy to replace batteries in cars, but I've seen claims that they are getting close.
A: Currently the best super-capacitors have an energy density (how much they can store in a given size) of only 10% of a rechargable battery.
But you can get the energy out of a capacitor very-very quickly, so the power available can 100 or 1000x as high as a batttery. So they have a very high power density.
This is the main use of capacitors in electronics, they provide a little 'holding tank' for electrical energy that is needed quickly by a device, say when an output switches, and the battery can't react as quickly. Bigger banks of capacitors are used to provide huge amounts of energy very quickly (and so high power) to fire large lasers or x-ray machines.
A: Supercapacitors don't get their large capacitance via their dielectric constant, which is usually about 10 - values of 10,000 are not uncommon for conventional capacitors. Instead, they get it from a super-thin dielectric - on order only a couple of Angstroms (2E-10 m). This means that for a rolled construction, each layer is very thin and so many layers can be packed into a regular sized package. This in turn means a huge surface area is available - as much as 10,000 square metres. This goes towards explaining how so much power is so readily available. The disadvantage of the thin dielectric is that only low voltages can be used, which in turn restricts the stored energy.
