A car battery is a type of rechargeable battery that supplies electric energy to an automobile. Usually this refers to an SLI battery (starting, lighting, ignition) to power the starter motor, the lights, and the ignition system of a vehicle’s engine. This also may describe a traction battery used for the main power source of an electric vehicle.

Automotive starter batteries (usually of lead-acid type) provide a nominal 12-volt potential difference by connecting six galvanic cells in series. Each cell provides 2.1 volts for a total of 12.6 volt at full charge. Lead-acid batteries are made up of plates of lead and separate plates of lead dioxide, which are submerged into an electrolyte solution of about 35% sulfuric acid and 65% water. This causes a chemical reaction that releases electrons, allowing them to flow through conductors to produce electricity. As the battery discharges, the acid of the electrolyte reacts with the materials of the plates, changing their surface to lead sulfate. When the battery is recharged, the chemical reaction is reversed: the lead sulfate reforms into lead oxide and lead. With the plates restored to their original condition, the process may now be repeated..

Lead-acid batteries for automotive use are made with slightly different construction techniques, depending on the application of the battery. The typical battery in use today is of the "flooded cell" type, indicating liquid electrolyte. AGM or absorbed glass mat type batteries have less liquid electrolyte than older style batteries and are gaining acceptance by consumers in SLI applications. This article deals with the flooded type of car battery.

The starting (cranking) or shallow cycle type is designed to deliver large bursts of energy, usually to start an engine. The SLI batteries usually have a greater plate count in order to have a larger surface area that provides high electric current for short period of time. Once the engine is started, they are recharged by the engine-driven charging system. See Jump start (vehicle).

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 specific energy is in the range of 30-40 watt-hours per kilogram.

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).

Some cars use more exotic starter batteries- the 2010 Porsche 911 GT3 RS offers a lithium-ion battery as an option to save weight over a conventional lead-acid battery.

Source: http://en.wikipedia.org/

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