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What is an AGM battery?

What is an AGM battery

Carmakers are switching to AGM battery types in late model cars and trucks to support start/stop systems. Many people don’t understand the difference between a standard lead acid battery and an AGM battery. Here’s what you need to know

What is an AGM battery

Traditional lead acid batteries and AGM batteries both use a mix of sulfuric acid and water as the electrolyte. However, in a traditional “flooded” lead acid battery the acid sloshes around. You would think that’s the best design to keep the acid in contact with the lead plates. But it’s not. When acid is added to an AGM it’s absorbed by a tightly packed fiberglass mats. Once the acid is absorbed by the mat, it doesn’t leak out.

cutaway view of an AGM battery What is an AGM battery

Notice the absorbent fiberglass mats between plates and separators.

Because the acid is always in contact with the plates, manufacturer’s have other options instead of just casting flat lead plates.

You may have seen Optima batteries with cylindrical cells. In that design the lead plate and glass mat are rolled into a cylinder like a sleeping bag. Many people call these “gel cell” batteries. They’re not. The acid is not a gel.

An AGM battery is a better battery

AGM batteries generally offer longer life, even if they’re run down to a deep discharge. In fact, an AGM battery can be discharged to 20% without causing permanent damage, as opposed to a flooded battery which gets into trouble at 50% or less.

They’re also more vibration resistant which is why they’re used in motorcycles.

They’re spill proof, so they can be installed in non-upright positions.

AGM batteries they don’t self-discharge as rapidly as a flooded cell.

Charges up to five times faster than a comparably sized flooded lead acid battery.

Performs better in cold weather when most flooded batteries slow down.

An AGM battery delivers more current than a flooded battery because it has lower internal resistance.

Now for the downsides of an AGM battery

• Easily and permanently damaged if overcharged or charged with a traditional battery charger. So you MUST use a charger specifically built to charge these types of batteries. That’s also why you shouldn’t jump to install and AGM style battery in a vehicle designed for a flooded battery. Most alternators have a charging rate of close to 14.4 volts. That’s too high for an AGM battery.

They’re more sensitive to heat

People think that cold weather is what kills most batteries. Wrong. It’s the heat. Sitting in extremely high underhood engine temps is what does them in. Some die in summer, but most hang on until winter, making you think the cold weather was the culprit.

But AGM batteries are even more sensitive to heat than flooded batteries.  That’s why more car makers are installing these batteries in locations away from the engine compartment.

They need more TLC

In addition to battery chargers that have an AGM setting, you also need battery special testing equipment. Due to their lower internal resistance, many computerized battery testers that don’t have a specific AGM testing routine will produce incorrect results. Load testing an AGM battery may also provide incorrect results.

What is AGM battery replacement cost?

They’re a lot more expensive than traditional flooded example of agm battery replacement cost. What is an AGM batterylead acid batteries. In late 2023, a group 35 lead acid battery sells for $184 at Batteries Plus stores. The same battery size in an AGM runs $259.

©, 2014 Rick Muscoplat

Posted on by Rick Muscoplat

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