Starting Battery

A starting battery is designed to deliver large bursts of power for a short time, as is needed to start an engine. Once the engine is started, the battery is recharged by the engine-driven charging system. Starting batteries are intended to have a low depth of discharge on each use. They are constructed of many thin plates with thin separators between the plates, and may have a higher specific gravity electrolyte to reduce internal resistance.
Starting batteries should never be discharged below 50% capacity.

Auxiliary Batteries

Auxiliary batteries are normally used for powering accessories such as fridges and lights. They can be located in the vehicle or in the trailer.
Auxiliary batteries are usually deep cycle batteries, which allows them to be discharged regularly to a lower level (30%) without reducing the life of the battery.

Battery Types

Lead Acid

Lead-acid batteries are made up of plates of lead and separate plates of lead dioxide, which are submerged into an electrolyte solution of about 35% sulphuric acid and 65% water.
Lead acid batteries should never be drained below 50% charge or the life of the battery will be reduced considerably.


Information to be added.


The absorbed glass mat (AGM) type uses a glass mat separator. AGM batteries differ from flooded lead acid batteries in that the electrolyte is held in the glass mats, as opposed to freely flooding the plates. Very thin glass fibers are woven into a mat to increase surface area enough to hold sufficient electrolyte on the cells for their lifetime. The fibers that compose the fine glass mat do not absorb nor are affected by the acidic electrolyte. These mats are wrung out 2–5% after being soaked in acids, prior to manufacture completion and sealing.

The plates in an AGM battery may be any shape. Some are flat, others are bent or rolled. AGM batteries, both deep cycle and starting, are built in a rectangular case to BCI battery code specifications.

AGM batteries will hold their charge very well and can still be 60% charged after 12 months.


A "gel cell" uses fine powder to absorb and immobilise the sulphuric acid electrolyte. These batteries are not serviceable: the cells are sealed so the degree of charge cannot be measured by hydrometer and the electrolyte cannot be replenished. A gel battery (also known as a "gel cell") is a VRLA battery with a gelified electrolyte; the sulphuric acid is mixed with silica fume, which makes the resulting mass gel-like and immobile. Unlike a flooded wet-cell lead-acid battery, these batteries do not need to be kept upright. Gel batteries reduce the electrolyte evaporation, spillage (and subsequent corrosion issues) common to the wet-cell battery, and boast greater resistance to extreme temperatures, shock, and vibration. Chemically they are almost the same as wet (non-sealed) batteries except that the antimony in the lead plates is replaced by calcium, and gas recombination could take place.
With gel electrolyte the separator was no longer such a critical, hard-to-make component, and cycle life was increased, in some cases dramatically. With gel electrolyte, shedding of active material from the plates was reduced.
More importantly, real gas recombination was used to make batteries that were not "watered" and could be called maintenance-free. The one-way valves were set at 2 psi, and this was high enough to have full recombination take place. At the end of charge when oxygen was evolved from overcharge on the positive plate it traveled through the shrink cracks in the gel directly to the negative plate made from high surface area pure lead and "burned" up as fast as it was made. This oxygen gas and the hydrogen adsorbed on the surface of the sponge lead metal negative plate combined to make water that was retained in the cell.
They are typically termed "maintenance-free" by proponents, or "unable to be maintained" by skeptics.
Gel batteries should not be mounted in the engine bay due to heat.

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