12 Volt battery used in vehicles is the Lead Acid (LA) type battery which can deliver more than 45 Ampere current to ignite the engine. It is the type of Starting battery or SLI battery (Starting Light Ignition) designed to give a heavy current to start the engine. The lead acid battery is composed of series of plates immersed in a solution of Sulphuric acid. Each plate has a Grid on which the active material is attached. In the negative plate, lead oxide is attached as the active material while on the positive plate, pure lead is attached. All the negative plates are connected together and connected to the external negative terminal. Similarly all the positive plates are connected together and with the positive terminal.
The electrolyte in the battery is sulphuric acid- water mixture which has specific gravity. The specific gravity is the weight of the sulphuric acid- water mixture compared to equal volume of water. Pure water has a specific gravity of 1.
During the battery discharge, the sulphuric acid in the electrolyte combines with the active material of the plates. This releases energy and the plate material becomes Lead sulphate both in the positive and negative plates. When this happens, hydrogen atoms of the sulphuric acid combines with oxygen to form water. As a result, positive plates release electrons and the negative plates accept equal number of electrons. This creates a potential difference between the positive and negative plates and current passes through the load.
During the charging and discharging process, heavy chemical reaction takes place which changes the specific gravity of sulphuric acid .This can damage the plates and may reduce the charging holding capacity. To prevent plate drying, topping with desired quantity of distilled water is necessary.