There are a few types of automotive 12 volt batteries. Most folks are most familiar with the SLI (starting, lighting, ingnition) battery. 12 volt car batteries have six galvanic cells inside stacked in  “series” fassion. Each cell provides 2.1 volts, together they equal 12.6 volts. A fully charged battery should read 12.6 volts. There are millions of tiny molecules that are constantly bouncing around in the battery. As the battery looses its charge these molecules stick to the lead plates in the battery. They are released when the battery is recharged. With the plates restored to their original condition, the process may now be repeated.

The deep cycle (or motive) type is designed to continuously provide power for long periods of time (for example in a trolling motor for a small boat, auxiliary power for a recreational vehicle, or traction power for a golf cart or other battery electric vehicle). They can also be used to store energy from a photo voltaic array or a small wind turbine. They usually have thicker plates in order to have a greater capacity and survive a higher number of charge/discharge cycles. The energy to weight ratio, or specific energy, is in the range of 30 Wh/kg (108 kJ/kg).

Batteries intended for SLI systems are intended to deliver a heavy current for a short time, and to have a relatively low degree of discharge on each use. They have many thin plates,thin separators between the plates, and may have a higher specific gravity electrolyte to reduce internal resistance. Deep-cycle batteries have fewer, thicker plates and are intended to have a greater depth of discharge on each cycle, but will not provide as high a current on heavy loads.

Some battery manufacturers claim their batteries are dual purpose (starting and deep cycling).

In normal automotive service the vehicle’s engine-driven alternator powers the vehicle’s electrical systems and restores charge used from the battery during engine cranking. When installing a new battery or recharging a battery that has been accidentally discharged completely, one of several different methods can be used to charge it. The most gentle of these is called trickle charging. Other methods include slow-charging and quick-charging, the latter being the harshest.

Batteries should be monitored and periodically charged if in storage, to retain their capacity. Batteries intended to be stored should be fully charged, cleaned of corrosion deposits, and left in a cool dry environment. High temperatures increase the self discharge rate and plate corrosion. Lead-acid batteries must always be kept in a fully charged condition. the terminal voltage can be measured as an indication of state of charge. Batteries may be charged periodically by a constant voltage method, or attached to a “float” charger.