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What is an AGM battery?

What is an AGM battery

If you’ve been shopping for a battery for your motorcycle, car, or lawn and garden tractor, you may have seen batteries listed as AGM (Absorbent Glass Mat) . If you’ve stopped to wonder what is an AGM battery, here’s the scoop.
Traditional lead acid batteries are filled with sulfuric acid and so are AGM batteries. 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 mat. Once the acid is absorbed by the mat, it doesn’t leak out. So AGM batteries are spill-proof, which is why they’re used in motorcycles.
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.

It’s 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 expensive motorcycles. Since the acid doesn’t leak, the battery can be installed in non-upright positions.  Plus, they don’t self-discharge as rapidly as a flooded cell. And, if you use the correct AGM charging mode on a newer style charger it charges up to five times faster than a comparably sized flooded lead acid battery.  It also performs better in cold weather when most flooded batteries slow down.
Given the same amount of lead, an AGM battery will deliver more current than a flooded battery because it has lower internal resistance. Sounds great, right? It is. ….until you get the bill. AGM batteries cost about twice as much as a comparable flooded battery. Gelled batteries cost even more.

Now for the cautions
AGM batteries can be easily and permanently damaged if overcharged or charged with a traditional battery charger. Gelled and other sealed style batteries are also sensitive to overcharging, 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.
Heat kills batteries
People think that cold weather kills most batteries. Wrong. It’s the heat. Sittings 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
I’ve already mentioned the special chargers. But you also need 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.

©, 2014 Rick Muscoplat

Posted on by Rick Muscoplat


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