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What is the battery/what does it do?

The Batteries typically used in vehicles powered by combustion engines are known as SLI type. This abbreviation stands for Starting, Lighting, and Ignition. The Battery is usually of a lead-acid type. It provides the high power required for the starter motor and also the power for all the electrical systems for a certain period when the engine is not running. When the engine is started and is running the alternator takes over the electrical supply and the battery is recharged.

Getting into the details of the battery

The lead-acid battery is a box shape, on average, approximately 25 cm x 20 cm x 18 cm in size and weighing on average 16KG.The box is constructed of heavy plastic, inside there are six cells with lead plates and lead oxide plates submerged in an electrolyte solution of sulphuric acid (38%) and water (62%).The cells are connected in series terminating in two lead posts/poles on the top of the unit one at each end. One post/pole is the positive connection and the other is the negative post/pole. The chemical reaction within each cell between the plates releases positive and negative electrons allowing them to flow through conductors to produce electricity. Each of the six cells delivers 2.1 volts for a total of 12.6 volts when the battery is fully charged. Commonly, there are two types of lead-acid batteries. The more modern battery is a no maintenance type, which is a sealed unit and has a life of between 2 and 5 years depending on quality. The second and older type is one where periodically checks are required of the electrolyte fluid level by removing the screw cap on top of each of the six cells and topping up with distilled water to ensure that the plates inside are kept well covered. Only water is needed as the acid remains constant as only the water evaporates during the chemical reaction. On this type the electrolyte fluid can be tested with a hydrometer to check the battery condition.

The battery in most cars with front engines is situated under the bonnet in the engine compartment. On some vehicles the battery is located under the rear passenger seats or sometimes in the boot compartment. The battery compartment has a base frame with clamps to secure it in position. There are two cables adjacent to the battery housing with bolt on clamp connectors made of copper and marked one + positive and one - negative these correspond to the two lead post/poles on the battery which are also marked + and - . Batteries are available with different post/pole positions so care must be taken to ensure that the post/poles on the new battery are in the right position to enable connection as the cables are only a certain length for safety reasons.

What if something goes wrong with the battery?

The battery has a relatively uncertain life span. Signs that it needs replacing can be that the starter motor is struggling and not turning at a normal speed. Causes for damage to a battery can be a failing alternator which is causing a failure in fully charging and the battery is being used constantly in the running of the engine and the electrical systems. Allowing the electrolyte fluid levels to drop too low in the cells also damages the battery. Loose connections also cause damage. Leaving lighting and electrical systems running without the engine operating can cause a deep discharge in the battery which can also damage it.