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Automobile Battery | Components & Working

The battery in an automobile is used as a chemical storage unit for the electrical energy produced by the alternator. The battery must be capable of delivering high current for the short duration for starting, and it must be able to furnish some or all of the electrical energy for other important system components for short period with the engine idle or off. Typically, system voltages are 12 V for passenger cars and 24 V for commercial vehicles (achieved by connecting two 12V batteries in series). The battery consists of following components:

1. Casing

The battery cases are generally made of synthetic resin. Recently, the most common material used is polypropylene. The casing is composed of a lid heat sealed to the molded bottom and sides.

2. Filler Caps

The battery filler caps are also made up of synthetic resin. A small hole is made in the cap to permit the escape of oxygen and hydrogen gases while containing the sulphuric acid in the electrolyte.

3. Battery terminals

The two terminals of the battery are made of lead alloy. The post-shaped terminals are slightly tapered to have easy connections of the battery leads.

4. Plates

A twelve-volt (12 V) battery is divided into six cells, each with several sets of positive and negative plates. Each plate is composed of a lead-antimony or lead-calcium alloy grid.

The lead peroxide on the positive plate is a collection of highly porous small brown particles. The lead peroxide breaks down into smaller particles over time and these are dislodged from the grid. The originally porous negative plate gradually loses porosity, contracts and becomes hard. In order to maintain the battery performance and porosity of the negative plate as long as possible, an expanding agent is added to the battery.

The separators are provided between the positive and negative plates in each cell to prevent short circuits between the cells. The separators must be nonconductive and porous while having sufficient mechanical strength and acid resistance. In addition to the separator, glass mats are formed on both sides of the positive plate to prevent loss of the active material and oxidation of lead peroxide.

5. Electrolyte

Each cell of the battery is filled with electrolyte or battery solution made up of two parts of pure sulphuric acid* and approximately four parts of distilled water. The contact of the electrolyte with the battery plates generates and stores electricity while improving electrical conductivity. The electrolyte of a fully charged battery contains about 31% sulphuric acid by weight or about 21% by volume in distilled water. This corresponds to specific gravity of 1.23 at 27°C. If impure sulphuric acid is added to the battery, the plates, separators, etc. will becomes corroded, resulting in an increase in self-discharging and shortened battery life. It is, therefore, vital that only pure sulphuric acid and distilled water or water purified by ion exchange be used.

Working of Battery

The working of battery is based on the principle that an electric current is generated when two kinds of metal having different ionization potential are connected with a conductive wire and submerged in an electrolyte. The electrical energy is converted into potential chemical energy during charging and during discharging, this potential energy is reconverted into electrical energy.

Discharge of battery

When the battery discharges, the positive plates containing lead peroxide (PbO2) and negative plate having spongy lead (Pb) combine with sulphuric acid (H2SO4) to form lead sulphate (PbSO4) and water (H2O). As discharging continues and the sulphuric acid continues to decompose, the concentration of sulphuric acid in the electrolyte gradually declines.

The amount of sulphuric acid used in the process is proportional to the amount of electricity drawn from the battery. This means that the specific gravity of the electrolyte reduces in proportion to the consumption of the battery electricity. The amount of electricity remaining in a battery can, therefore, be determined by measuring the specific gravity of the electrolyte.

If discharging continues beyond the limit, the lead sulphate (PbSO4) formed on plates forms into white crystals. This is called sulphation. At this point, the chemical reaction is irreversible and the battery cannot be recharged. For all intents and purposes, the battery is "dead".

Charging of battery

When a current from a DC power source (alternator) is applied to a discharged battery, the lead sulphate on the plates is electrochemically decomposed. In this process, Sulphur ions are emitted by the plates and the specific gravity of the electrolyte is increased. At the same time, active material on plates return to their original state, i.e., to lead peroxide and to spongy lead, restoring the battery to full function.

During the final stage of battery charging, the emission of Sulphur ions from the plates ceases and at the same time, electrolysis of water in the battery begins. The hydrogen gas is emitted around the negative plate and oxygen gas is emitted around the positive plate.

As a battery is fully charged, the voltage rises gradually. When the battery begins to give off gases, the voltage rises rapidly to the maximum stabilized level. When fully charged, each cell has 2.1 to 2.2 volts. The specific gravity of the electrolyte also increases gradually during charging until gases are given off because there is a little agitation of the fluid. When the gas emission begins, the specific gravity increases rapidly, reaching the maximum level at the end of charging and remaining constant thereafter.

* Sulphuric acid (chemical formula H2SO4) is a highly corrosive and poisonous acid. Sulphuric acid is soluble in water and reacts with most metals. The melting point is 10° C. This acid is widely used in chemicals, fertilizers, explosives, oil refining etc. Since it has a dehydrating property, therefore, paper and other fibers become carbonized and black when they come in contact with the acid.


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