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 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.
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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.