A capacitor is like a battery.
This is like saying a compressed air
tank is like an air compressor. A compressed air tank will lose
pressure quickly as you use the air stored inside it. An air compressor
will supply air at a steady pressure as long as you supply power. A
charged capacitor will act like the compressed air tank. As you use it
the voltage will drop rapidly. A battery has a large energy store in
its chemical makeup and will supply a steady voltage many, many times
longer. EDLCs are nearly an exception. They have such large capacities
that they rival batteries in energy density in some applications.