Absorbent glass mat

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Absorbent glass mat (AGM) is a class of lead-acid battery in which the electrolyte is absorbed into a fiberglass mat. It is one of the types of VRLA battery. The plates in an AGM battery may be flat like wetcell lead-acid batteries, or they may be wound in tight spiral. Their unique (for lead acid chemistries) construction also allows for the lead in their plates to be purer as they no longer need to support their own weight as in traditional cells. Their internal resistance is lower than traditional cells, they handle higher temperatures better, and self discharge more slowly. These batteries have a pressure relief valve which will activate when the battery is recharged at high voltage (typically greater than 2.30 volts per cell, a 12 volt battery has 6 cells). Valve activation allows some of the active material to escape thus decreasing the overall capacity of the battery. The lids (covers) typically have gas diffusers built into them that allow safe dispersal of any excess hydrogen that may be formed during over-charge. They are not permanently sealed, but are maintenance free; and they can be oriented in any manner, unlike normal lead-acid batteries which must be kept upright to avoid acid spills and to keep the plates' orientation vertical. Their specific power is very good so they can be charged and discharged quite rapidly, however their specific energy tends to be lower than traditional flooded golf cart batteries. They are often used in high performance electric vehicles due to their high power density.

In cylindrical AGM's, the plates are thin and wound, like most consumer disposable and rechargeable cells, into spirals so they are also sometimes referred to as spiral wound.

AGM, rechargeable batteries usually come in large sizes.

The Mazda Miata sports car uses an AGM battery. Because the battery is situated in the trunk (boot) the consequences of acid spillage following an accident would be more severe. The AGM battery dramatically lowers this risk.