What is a battery?

A battery is a device for storing electrical energy in a chemical form, and then releasing it as direct current in a controlled way. All types of batteries contain a positive and a negative electrode immersed in an electrolyte, the whole assembly being within a container. All Electra are lead-acid batteries, which mean that they have positive and negative electrodes made of lead compounds in a dilute sulphuric acid electrolyte. Lead-acid batteries are secondary batteries, which mean that they can be recharged after they have been discharged. Primary batteries can be discharged only once and then have to be thrown away; examples are some types of torch and radio batteries.

What is a Battery Made of?

Grids
As the positive and negative electrodes are made of weak materials, they need a mechanical support which is provided by a grid made from a lead alloy; lead on its own would be too soft. In addition to providing a support for the electrodes (the active material), the grid also conducts electricity from the electrodes to the outside load.

Electrodes
The electrodes are initially made from a mixture of lead oxide and lead sulphate. This is converted into lead dioxide in the positive plate and porous lead in the negative plate when the battery is initially charged. The negative electrode also contains small amounts of additives to give the battery a good discharge performance at low temperatures to improve starting. The combination of grid and electrode is normally called a plate.

Electrolyte
The electrolyte is dilute sulphuric acid. This acts as a conductor to transport electrical ions between the positive and negative plates when the battery is being charged or discharged. The acid also takes part in the discharge as the sulphate ions react chemically at the electrodes to produce lead sulphate.

Separator
The separator is an insulator placed between the positive and negative plates to prevent them shorting together. The separator needs to be microporous with very small holes to allow the ions to flow through the separator from one plate to another. It also needs to be able to resist the high temperatures and strongly acidic oxidising conditions that occur in a battery. Most modern separators are made of microporous polyethylene, which has the right properties to meet the demanding conditions within the battery.

Container and Lid
These are normally made of polypropylene, which is a light but strong plastic. Unlike some plastics, it does not become brittle when it is cold, and so can resist knocks during handling. It is not attacked by acid and it can also withstand the fluids (petrol, diesel, brake-fluid, antifreeze) normally found on a vehicle.

How do Battery Work?

The positive electrode is made of lead dioxide and the negative electrode is made of porous lead. When an electrical load (for example lights or a starter-motor) is connected across the battery, a current flows through the electrolyte in the battery and through the external load. This causes the battery to discharge, which results in the chemical composition of both the electrodes changing to lead sulphate. A battery can be charged by putting a current through the battery from an outside source of electricity such as an alternator, dynamo or charging unit. This converts the lead sulphate back to the original materials of lead dioxide and porous lead. As the battery becomes charged, the electricity begins to decompose (hydrolyse) the water in the electrolyte into its constituent elements of hydrogen and oxygen, which are released as gas. This is why a battery gases when it is charged.