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What you need to know about AGM batteries

AGM Battery Diagram Photo by Lifeline Batteries

In recent years, battery manufacturers have developed a new, more robust type of lead-acid battery known as an absorbent glass mat (AGM) battery.

What makes them different

An AGM battery is built around a fine, highly absorbent fiberglass mat sandwiched between plates saturated with battery acid. Because the plates and mats are packed tightly, AGM batteries are basically immune to vibration. The terminals on AGM batteries are less likely to corrode, and AGM batteries also can be recharged faster than conventional lead-acid batteries.

AGM batteries last longer than conventional lead-acid batteries and discharge more slowly when not in use. Additionally, the glass mats in AGM batteries aren’t fully saturated, so the liquid can’t expand and cause plate and case damage if the battery is exposed to subfreezing conditions.

Finally, AGM batteries are useful in applications that require a capacity for deep cycling, such as engine stop-start systems. The continuous draining and recharging of start-stop systems, used in an increasing number of cars, causes a sharp fade in a conventional lead-acid battery’s capacity in as little as 2 years of use.

Before you go out and get one

Many vehicles made in the past decade—especially luxury cars with high electrical demands—are installing AGM batteries as original equipment. But because AGM batteries are 40% to 100% more expensive than conventional batteries, a good lead-acid battery might be more cost-effective if an AGM battery’s advantages don’t apply to a specific application.

In many cases, a vehicle’s OEM lead-acid battery can be replaced with an AGM battery, provided the vehicle manufacturer approves it. However, it might be unwise to replace an original-equipment AGM battery with a standard lead-acid battery, because the charging system might be optimized for an AGM battery, and the vehicle’s electrical equipment might not run as efficiently with a non-AGM battery.

Motorists interested in replacing a conventional battery with an AGM battery should consult a trusted technician at their dealership or independent shop before making the switch.

Excerpted and adapted from the AAA Car Guide, available in hard copy at AAA branches.

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