|MadSci Network: Engineering|
Your theory is basically correct.
You can use a battery to charge a capacitor, and then that capacitor can be discharged into a load.
When capacitators are used in things like rectifiers they are in fact doing just what you say. They store and then discharge energy to maintain a more even output voltage.
The charging and discharging rate of a capacitor depends on the size of the capacitor and the in series resistance. This is called the time constant. Have a look at this website for a calculator in which you can vary R and C and see what sort of time constants you can expect. You will see that for a low resistance devices you would need a pretty big capacitator to maintain a current flow for seconds. See: Charging a Capacitor
This site: http://www.electronics2000.co.uk/ has some general background theory on capacitors
Try the links in the MadSci Library for more information on Engineering.