Why water topping is essential in Car battery? Fact File 31

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.

Battery and Cell

A lead acid battery is composed of array of cells. This array of cells is known as Battery. A 12 volt Mono block battery has 6 cells (each gives 2 volts) but a 2 volt tubular battery has only one cell.

The capacity of the battery to deliver charge is expressed in terms of Ampere hours (Ah). One Ah = 3600 coulombs. 1 Ah battery can provide 1 ampere current in 1 hour. The 100 Ah tubular battery used in Inverters can give 1 ampere current for 100 hours. Battery rating is based on the constant discharge rate for a particular time; say 10 hours, till the terminal voltage reduces to minimum. Typically a 100 Ah battery gives 5 ampere discharging current for 20 hours. During charging, the energy from the external power source causes the chemical changes in a reverse manner so that battery regains its charge.

Battery cycle

One complete discharge and recharge forms a battery cycle. This cycle is usually a discharge from 100 % to 20 % change followed by recharging from 20 % to100 %. Life of the battery depends on the number of deep cycles per day. For example, if a battery is discharged 50 % per day and recharged to 100 %, its life increase to twice than a battery discharging and recharging occasionally. Therefore it is better to discharge and recharge emergency lamp battery and inverter battery daily to increase its life. It is also better to start vehicles daily even if it is not running. This prevents self discharge and increases the efficiency for cycling.
Self discharge.

If the lead acid battery is not discharging through the load, self discharge takes place @ 4 % per week at 27 degree. For example, a 125 Ah battery shows self discharge @ 5 Amps current per week if it is not discharging through the Inverter.