A car battery is actually 6 smaller batteries that are lined up in series. This causes the voltages of each battery to add.
The general way that a battery works is that when an electronic circuit is connected to the battery, electrons are allowed to flow. Inside the battery are 3 important things. There are 2 connectors that go out of the battery. These are called the cathode and anode. There is also a solution that the cathode and anode sit in. During normal operation, a chemical reaction occurs between the solution and the anode which releases electrons that flow through the circuit. These reenter the battery through the cathode where another chemical reaction is happening between the cathode and solution. The electrons are incorporated in the products of this reaction. When run in reverse (with certain batteries), electrons are forced in the other direction in the reverse reactions. When a battery dies, it is because one or more of the chemical reactants is more or less used up.
In a car battery (sometimes called a lead-acid battery) the cathode is lead dioxide (PbO2)
, the anode is a sponge of lead (Pb), and the solution is sulfuric acid (H2
). When the battery is being used, the 2 connections react to form lead sulfate (PbSO4
) by reacting with the sulfuric acid.
Specifically, the two reactions are:
+ 4H+ + SO4--
Pb + SO4--
Notice that one reaction releases electrons and the other uses them up. Since the electrons can't travel directly through the solution to get from where they're released to where they're used, the reactions can only continue if they travel through the external electrical circuit.
The reverse reaction happens when the battery is being recharged. This reaction produces roughly 2V. The 6 cells add to give the 12V necessary to start a car.
(republished on 07/13/06)