A vehicle is generally using as much electricity as the alternator is generating, there are times when extra energy is needed, and that is one of the times when a battery is useful. This is especially the case when starting the engine, which requires energy to get the engine going (and that energy comes from the starter motor, which is powered by the battery).
Sometimes the alternator generates more electricity than the vehicle needs to use, and this extra electricity recharges the battery.
How do Automotive Batteries Store Energy?
No battery actually stores electricity, they instead store potential energy (chemicals which can be converted into energy and other chemicals without potential energy) which can be used later. Car batteries are one of the types that can recharge, and this is due to the chemicals stored within it. Car batteries are typically lead-acid batteries consisting of six sets of two-volt cells in series which form a 12-volt system.
In a lead-acid battery, plates of lead (as seen in the picture at the right) are submerged in a solution of about 35% sulfuric acid and 65% water. When electrons are discharged as electricity, the sulfur chemically bonds to the surface of the lead plates as lead sulfate. When the battery is recharged, this process is reversed.
Cold Cranking Amps
The CCA (cold cranking amps) rating is about how many amps the battery has (when fully charged) to turn over the starter at 0°F (-18°C) for 30 seconds. This rating is most widely used to compare batteries because it is one of the more difficult tests for a battery to score well on.
Also called MCA on marine-grade batteries, the cranking amp rating is the same as the CCA rating, except at 32°F (32°C). Cranking amps aren't used as much as cold cranking amps for quality comparisons since they are usually closer together and always in a useful range.
Reserve Capacity Minutes
A measurement of how much energy is stored in a battery is RCM (Reserve Capacity Minutes), which refers to how long a battery will continuously deliver 25 amps before its voltage drops below 10.5 volts.