I know that the main difference between the two is that a battery can provide a constant voltage whereas a capacitor's voltage decreases as the charge stored decreases. But what about the internal structure ? A battery also has chemical reactions going inside of it while a polarized capacitor also has an electrolytic structure. So is there or is there not any difference between the internal workings of a polarized capacitor and a battery?
Batteries produce a charge difference across the terminals as a result of a chemical reaction. A chemical gets changed into another one. Even in a rechargeable battery a chemical change takes place that is reversed.
An electrolytic capacitor uses chemistry to create a thin layer with an electric field across it. The thinner layer than a "regular" capacitor allows more charge to be stored in a smaller space. But the fundamental difference is that there is no chemical change when a capacitor is charged
$\begingroup$ and a capacitor once it loses its charge no watthours to replace it. In other words a battery can charge a capacitor but a capacitor cannot charge a battery :). $\endgroup$ Nov 22, 2013 at 10:08
$\begingroup$ @annav, a charged capacitor can at least partially charge a battery, i.e., a discharging capacitor supplies power to the attached circuit whether the circuit is a resistor or a battery. $\endgroup$ Nov 22, 2013 at 14:01
$\begingroup$ @AlfredCentauri true, but very transient, no wattshours $\endgroup$ Nov 22, 2013 at 14:05
$\begingroup$ @annav, for example, this 3000F 2.7V capacitor stores 3Wh at rated voltage: maxwell.com/products/ultracapacitors/docs/… $\endgroup$ Nov 22, 2013 at 14:21
$\begingroup$ @AlfredCentauri my point is that a battery stores appreciable energy for use when needed. A capacitor is not a good choice for energy storage. $\endgroup$ Nov 22, 2013 at 14:24