Automotive Research

The positives & negatives of advanced AGM batteries

Car starter batteries on sale in a store

Conventional flooded lead-acid starter batteries face a challenge on today's roads.

Increasingly, cars and light trucks—particularly hybrids—are equipped with engine start-stop systems, which turn off the engine when the vehicle is at rest and restart it when the driver releases the brake pedal. Conventional starter batteries aren’t well-suited to such repeated use, however, and often suffer a sharp drop in capacity in as little as 2 years of stop-start use.

In recent years, auto-parts manufacturers have responded to the need for a more robust battery with the development of a new type of lead-acid battery known as an absorbent glass mat (AGM) battery. We look at what makes AGM batteries, and whether you need one. 

How AGM batteries are different

Jet on a aircraft carrier

AGM technology has been around since the 1980s; originally used in military applications, it’s been adapted for use in passenger vehicles only in recent years. 

AGM batteries are constructed differently from the typical OEM flooded lead-acid batteries used in most vehicles. AGM batteries are built around a fine, highly absorbent fiberglass mat sandwiched between plates that are saturated with battery acid. (In conventional batteries, plates are instead submerged in a pool of free-flowing acid.) The plates can be made flat, wound into cylindrical cells, or configured in other ways.

Car battery cross section

The glass mat absorbs and immobilizes the acid electrolyte while still keeping it evenly distributed across plate surfaces. As a result, even if the battery is broken, no acid will spill. And because the plates and mats are packed tightly, AGM batteries don't really suffer deterioration from vibration.
Additionally, the terminals on AGM batteries are less likely to corrode, which means less maintenance is required. AGM batteries also have lower internal resistance than conventional lead-acid batteries, which enables them to be recharged faster. Although generally described as maintenance-free, AGM batteries still require cleaning and regular functional testing.

Advantages of AGM batteries

  • AGM batteries typically last longer than conventional batteries and discharge more slowly when not in use, so they’re beneficial in vehicles that aren’t driven for long periods at a time.
  • They're especially useful in applications that require a capacity for deep cycling, such as engine stop-start systems.
  • The glass mats in AGM batteries aren’t fully saturated, so the liquid can’t expand enough to cause damage if the battery is exposed to sub-freezing conditions.
  • AGM batteries are virtually leak-proof and highly resistant to shock and vibration, so they’re a good choice where a leaking battery could create acid damage.
  • They emit much less hydrogen than other batteries, so they offer an advantage in poorly ventilated areas, where fumes might create a hazard.

Disadvantages of AGM batteries

  • AGM batteries cost more than conventional batteries—in most cases, between 40% and 100% more. For cars that don't use stop-start technology, a good flooded lead-acid battery might be more cost-effective.
  • AGM batteries don’t tolerate extreme heat well; in such cases, a flooded battery may perform better if the water levels are checked regularly.
  • AGM batteries are sensitive to charging voltage; they sometimes dry out in very hot conditions, especially if the charging voltage is high.
  • A deep discharge can cause an AGM battery's voltage to drop very low, into single digits, though the battery can usually be recovered if it is otherwise healthy.
Car battery

When to look at an AGM battery

  • If your vehicle came with a conventional battery, it can probably be replaced with an AGM battery. Owners of cars with start-stop engines and conventional batteries in particular should consider an AGM battery.
  • If your vehicle originally came with an AGM battery, it is not advisable to swap in a conventional battery. The vehicle's charging system may be optimized for AGM batteries and could run poorly on a conventional one.

Interested in switching to an AGM battery?

Swapping out your existing conventional starter battery for a more efficient AGM battery could pay off, depending on the kind of vehicle you drive and the conditions it faces. Get an expert's advice by stopping at a AAA Approved Repair shop.

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