Knowledge What Is a Car Battery


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A car battery is a rechargeable battery that is designed for use in automotive SLI (Starting, Lighting and Ignition) applications. This type of battery has high momentary current capability required to start an internal combustion engine (ICE) of a car, truck, boat, etc. The most important function of the battery is to deliver quick bursts of power to crank and start the engine. In addition, the battery supplies power for the lighting systems, on-board electronics and electrical accessories when the engine is not operating. The battery acts as a voltage stabilizer for the entire electrical system and provides current whenever the electrical load of the vehicle exceeds the output of the charging system.

All automotive SLI batteries currently harvest the lead-acid chemistry which offers the proven dependability as well as a price-to-power ratio superior to all other chemistries. The electrochemistry of lead-acid batteries uses a lead dioxide (PbO2) plate as the positive electrode (cathode), a lead (Pb) plate as the negative electrode (anode), and a sulphuric acid electrolyte. In the discharge state, the lead plate undergoes an oxidation reaction which frees electrons to conduct current through an external circuit to the lead oxide plate which undergoes a reduction reaction. Both the electrodes are converted to lead sulphate (PbSO4). This causes the sulfuric acid in the electrolyte to be consumed. The cycle is reversed in the charge state. When a charging voltage is applied, the lead sulfate and water are electrochemically converted to the original lead dioxide, lead and sulfur acid.

There are two general categories of lead-acid batteries that are commonly seen in automotive SLI applications: flooded lead-acid (FLA) batteries and valve-regulated lead-acid (VRLA) batteries. FLA batteries have vent caps that allow for the escape of the hydrogen and oxygen gases generated during charging. VRLA batteries are sealed to facilitate oxygen recombination reactions. A one-way, pressure-relief valve is utilized to eliminate excess pressure from the chemical reactions. VRLA batteries are further divided (by their technique for immobilizing the electrolyte) into absorbed glass mat (AGM) batteries and gel batteries.

A car battery is constructed with a large number of thin plates. This construction increases the effective surface area available for energy storage and allows the battery to have low internal resistance. As a result, a car battery is able to deliver high power pulses for engine startup. However, this type of battery is limited to 20% of capacity discharge. When the car battery is used for deep cycle applications it will generally fail after 30-150 deep cycles. This is because the thinner plates are more susceptible to warping and pitting when deeply discharged. On the other hand, the battery may last for thousands of cycles if it is shallowly discharged (2-5% discharge).

A battery's ability to provide high currents with stable voltage is inversely proportional to the ambient temperature. Therefore car batteries are rated in Cold Cranking Amps (CCA) which indicates the amount of current a battery can deliver for 30 seconds at 0°F (-17.8°C) while maintaining a voltage of 1.2 volts per cell (7.2 volts per battery) or higher.