A car battery is a physical reserve of energy within the car system. The battery is usually charged by an alternator — a device that converts the mechanical energy from the engine into electrical energy. This energy is then used to prompt reactions within the electrolyte of the battery, which is most commonly composed of lead oxide plates submerged in a mixture of sulfuric acid and water. When a car battery is in use, the sulfuric acid reacts with the lead oxide plates, to form lead sulfate. Recharging the battery involves reversing this reaction. In this way, the car battery is able to store energy chemically.

The car battery is used to power up a car during ignition and provide energy for the lighting system. In petrol engines, it is also used to ignite the fuel; petrol engines, therefore, can not work without these auto batteries. Diesel engines may work without batteries, if the initial electrical energy is provided at ignition. Nowadays, the car battery is also being used to provide traction energy for the electric vehicles.

There are several types of car batteries available now. The most common is the flooded type, which is basically an arrangement of lead plates immersed in sulfuric acid in a pseudo-open reservoir. This type requires regular topping up with distilled water to compensate for the water lost as hydrogen and oxygen gases during the electrolysis process. A slight modification of this results in a different type of battery known as a sealed battery, which is basically a flooded battery which is sealed so that no water can be lost.

A valve regulated lead acid (VRLA) battery is also sealed. One type has safety valves that allow for the safe escape of hydrogen and oxygen gases generated during charging. There is another type of VRLA battery that doesn't have safety valves and instead has an additional catalyst surface on which the two gases recombine to form water again.

The glass mat kind of car battery is radically different from the VRLA, sealed and flooded types of batteries. Its composites are boron silicate glass mats saturated with electrolyte between the plates. By using the recombinant technology, they don’t lose hydrogen or oxygen. They also store charge for a much longer period, losing only 1% to 3% monthly. However, they are usually two to three times more expensive than the normal flooded batteries. Another group of battery are silica gel batteries, but due to their slow recharge rates, they are not common in cars.

Two other electrolytes are used less commonly in automotive batteries. One is the Nickel metal Hydride (NiMH), which packs a relatively higher energy within a unit of space. This is also referred to as high energy density. NiMH batteries can be easily recycled, since they do not contain toxic metals. Lithium-ion (Li-ion) batteries also has a high energy density. They also self discharge the least over a period of time.

When storing sulfate batteries, it is important to ensure that they are always charged, preferably to the maximum. Discharged sulfate batteries tend to degrade by sulfating. The degradation involves formation of large lead-sulfate crystals in the batteries, which hinders the normal reverse reaction that defines recharging. Store the batteries in a cool place, as warm temperatures result in a higher rate of discharge and corrosion at the contact plates.