AGM technology became popular in the early 1980s as a sealed lead acid battery for military aircraft, vehicles and UPS to reduce weight and improve reliability. The sulfuric acid is absorbed by a very fine fiberglass mat, making the battery spill-proof. This enables shipment without hazardous material restrictions. The plates can be made flat to resemble a standard flooded lead acid pack in a rectangular case; they can also be wound into a cylindrical cell. What is an agm battery?

What is an agm battery?

An AGM (absorbed glass mat) battery contains a special glass mat separator that wicks the electrolyte solution between the battery plates. This material’s design enables the fiberglass to be saturated with electrolyte – and to store the electrolyte in a “dry” or suspended state rather than in free liquid form. As the agm battery works, electrolyte is transferred from the glass mat to battery plates as required. The mat contains enough electrolyte for the battery to deliver its full capacity and – should the battery case become damaged or the battery is tipped on its side – its electrolyte will not spill.

AGM (Absorbed Glass Mat) sealed battery technology was originally invented in 1980, and developed and introduced in 1985 for military aircraft where power, weight, safety, and reliability were paramount considerations.

what is an agm battery

AGM is the preferred battery for upscale motorcycles. Being sealed, AGM reduces acid spilling in an accident, lowers the weight for the same performance and allows installation at odd angles. Because of good performance at cold temperatures, AGM batteries are also used for marine, motor home and robotic applications.

AGM is making inroads into the start-stop function of cars. The classic flooded type is simply not robust enough and repeated cycling causes a sharp capacity fade after only two years of use.

As with all gelled and sealed units, AGM batteries are sensitive to overcharging. A charge to 2.40V/cell (and higher) is fine; however, the float charge should be reduced to between 2.25 and 2.30V/cell (summer temperatures may require lower voltages). Automotive charging systems for flooded lead acid often have a fixed float voltage setting of 14.40V (2.40V/cell); a direct replacement with a sealed unit could overcharge the battery on a long drive.

AGM battery – Deep Cycle Or Not?

An important thing about AGM – Just because a battery is AGM does NOT make it a deep cycle battery. Several companies have adopted AGM for starting batteries and other non-deep cycle applications. Those still have the advantages of AGM battery, but are not deep cycle. It is primarily plate thickness that makes a battery deep cycle, not whether it is flooded, gelled, or AGM.

Battery Efficiency

This comparison is important and is critical for high charge or discharge rate applications. Internal resistance of a battery denotes its overall charge/discharge efficiency, its ability to deliver high currents without significant drops in voltage, and is a measure of the quality of the components and construction.

Battery Internal Resistance Losses

Losses from internal resistance shows up as heat, which is why batteries tend to get warm when heavily charged or discharged for a while.

Internal resistance losses in standard flooded Lead-Acid batteries is usually around 10% to 15% for a new battery, and can be as high as 25%+ for older batteries. This can vary considerably, depending mainly on the age and quality of the battery. In general, you get what you pay for – cheaper batteries with thinner plates and internal connections tend to be less efficient.

How agm battery works

The superpowers of an AGM battery come from two novel additions to Planté’s invention and a host of small design changes that fundamentally expand what car batteries can do.

First, a valve prevents evaporated water from leaving the battery case. This might not sound like much more than the inverse of the one-way valves on coffee bean bags.

How agm battery works

But this little trick is the secret to an AGM’s long life. Here’s how.

Its fundamental chemistry is still based on lead, sulfuric acid and water. When you draw power, the acid molecules move to the lead plates, leaving water and lead sulfate. You are removing the sulfuric acid from the solution to enable a chemical reaction between the paste on the plates. This process is reversed when you charge the battery.

However, there’s always a chance some water loss can happen when electricity splits H2O into hydrogen and oxygen gases. Losing those water molecules means the electrolyte stays more acidic than usual — cutting into the potential strength of the chemical reaction on the plates, and ultimately shortening the life span.

The AGM’s valve stops those gases from leaving.

Except if you’re overcharging the battery. When you use the wrong charger for an AGM, the current must pass through anything it can. That means breaking up more water molecules and building up too much gas inside. That’s when the safety mechanism kicks in, releasing some gas to reduce the pressure built up inside the battery.

Second: fiberglass mesh mats. They are the GM in AGM (absorbed glass-mat) batteries.

Ultra-thin glass fibers soak up all the electrolyte (water and sulfuric acid) into thin pillows cushioning the lead plates. Instead of the free-flowing liquid inside of a regular car battery, the AGM carries its charge in soaked sponges coating the lead plates. The glass mats’ complete coverage makes it easier to summon more power from an AGM battery — and make it easier to recharge.

In power, speed, long life and durability, the AGM battery has standard batteries beat.

Which agm battery is best?

AGM and other sealed batteries do not like heat and should be installed away from the engine compartment. We will recommend halting charge if the battery core reaches 49°C (120°F).

You can see the advantages and limitations of AGM battery as below:

AdvantagesSpill-proof through acid encapsulation in matting technologyHigh specific power, low internal resistance, responsive to loadUp to 5 times faster charge than with flooded technologyBetter cycle life than with flooded systemsWater retention (oxygen and hydrogen combine to produce water)Vibration resistance due to sandwich constructionStands up well to cold temperatureLess prone to sulfation if not regularly topping charged

Has less electrolyte and lead than the flooded version
LimitationsHigher manufacturing cost than flooded Sensitive to overcharging (AGM has tighter tolerances than gel)Capacity has gradual decline (gel has a performance dome)Low specific energyMust be stored in charged condition (less critical than flooded)


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