What is a Battery Made of

Battery Construction
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.

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.

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.

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.