An automotive battery is a rechargeable battery that supplies electrical current to a motor vehicle. Its main purpose is to feed the starter, which starts the engine. Once the engine is running, power for the car's electrical systems is supplied by the alternator.

Typically, starting discharges less than three percent of the battery capacity. For this reason, automotive batteries are designed to deliver maximum current for a short period of time. They are sometimes referred to as "SLI batteries" for this reason, for Starting, Lighting and Ignition. SLI batteries are not designed for deep discharging, and a full discharge can reduce the battery's lifespan.

As well as starting the engine, an SLI battery supplies the extra power necessary when the vehicle's electrical requirements exceed the supply from the charging system. It is also a stabilizer, evening out potentially damaging voltage spikes. While the engine is running, most of the power is provided by the alternator, which includes a voltage regulator to keep the output between 13.5 and 14.5 V. Modern SLI batteries are lead-acid type, using six series-connected cells to provide a nominal 12 volt system (in most passenger vehicles and light trucks), or twelve cells for a 24 volt system in heavy trucks or earth-moving equipment, for example.

Battery electric vehicles are powered by a high-voltage electric vehicle battery, but they usually have an automotive battery as well, so that they can use standard automotive accessories which are designed to run on 12 V.

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Excess heat is a main cause of battery failures, as when the electrolyte evaporates due to high temperatures, decreasing the effective surface area of the plates exposed to the electrolyte, and leading to sulfation. Grid corrosion rates increase with temperature.

A vehicle with a flat battery can be jump started by the battery of another vehicle or by a portable battery booster, after which a running engine (but running faster than idle speed) will continue to charge the battery.

Corrosion at the battery terminals can prevent a car from starting due to electrical resistance, which can be prevented by the proper application of dielectric grease.

Sulfation is when the electrodes become coated with a hard layer of lead sulfate which weakens the battery. Sulfation can happen when battery is not fully charged and remains discharged.Sulfated batteries should be charged slowly to prevent damage.

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SLI batteries are not designed for deep discharge, and their life is reduced when subjected to this.

Car batteries using lead-antimony plates require regular topping-up with pure water to replace water lost due to electrolysis and evaporation. By changing the alloying element to calcium, more recent designs have reduced the rate of water loss. Modern car batteries have reduced maintenance requirements, and may not provide caps for addition of water to the cells. Such batteries include extra electrolyte above the plates to allow for losses during the battery life.

Some battery manufacturers include a built-in hydrometer to show the state of charge of the battery.

 

A positive (red) jumper cable connected to battery post. An optional hydrometer window is visible by the single jumper clamp. The black negative jumper clamp is not shown.

The primary wear-out mechanism is the shedding of active material from the battery plates, which accumulates at the bottom of the cells and which may eventually short-circuit the plates. This can be substantially reduced by enclosing one set of plates in plastic separator bags, made from a permeable material. This allows the electrolyte and ions to pass through, but keeps the sludge build up from bridging the plates.