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What is an AGM Battery? How They Work, Pros & Cons

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From military aircraft to cutting-edge start-stop vehicles, the AGM battery has come a long way! 

But what sets it apart from the rest? 
And what are its benefits and drawbacks? 

In this article, we’ll dive deep into what AGM batteries are, how they work, their pros and cons, and answer all your burning questions. We’ll also drop an easy solution to any AGM battery issues you may be facing.

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Let’s get started.

What’s an AGM Battery?

The AGM battery is a sealed lead acid battery (also known as a Valve Regulated Lead Acid or VRLA battery). 

AGM is short for Absorbent Glass Mat, which refers to the battery technology used. Developed in the early 80s, absorbed glass mat AGM batteries were first applied to military aircraft and designed as an alternative to NiCad (NiCd) batteries which were very costly. 

Today, you’ll find this deep cycle AGM battery in all demanding applications — from marine to aviation, emergency lighting, and even off-grid power systems like wind and solar. An absorbed glass mat battery is also well-suited to advanced cars with start-stop technology and significant power demands. 

AGM batteries are especially known for their high reserve capacity, which allows them to provide a steady power supply for longer durations.

Owing to these inherent benefits and uses of AGM batteries, the AGM battery market value is set to grow by almost $18 billion from 2022 to 2032 at a 4.8% CAGR. Eminent global AGM battery market players include Clarios, Power Sonic Corporation, Crown Battery, Exide Technologies, C&D Technologies, and East Penn Manufacturing Company.

Let’s now go through the inner workings of the absorbed glass mat AGM battery.

How Do AGM Batteries Work?

The secret to an AGM battery’s enhanced performance lies in an ultra-thin glass mat, sometimes called an AGM separator.  

The fiberglass mats in AGM sealed batteries are sandwiched between a lead plate on each side, soaking up the electrolyte. It absorbs and holds the battery acid. This way, the electrolyte is suspended in a “dry” state instead of the free-flowing form found in a traditional flooded acid battery. The glass mat maximizes the surface area for the electrolyte to react with the battery plates. This allows a fast reaction between the battery acid and plate material. 

Remember that this is a VRLA (Valve Regulated Lead Acid) battery. 

What does the “valve” do?
The AGM battery electrolyte is typically a mix of sulfuric acid and water. While charging, the chemical reaction produces hydrogen and oxygen. The battery valve stops the gases from leaving, preventing water loss. These gases are reabsorbed into the electrolyte. 

However, the valve vents the gases when there’s too much pressure (like when the battery is overcharged.) This helps equalize pressure and prevents structural damage to the battery. 

Next, let’s see how AGM technology influences the battery’s features.

7  Advantages of AGM Batteries

AGM technology batteries feature a host of benefits. 
Here are the advantages:

1. Longer Battery Life

An AGM battery has a longer lifespan than a conventional battery (lead acid battery) — lasting up to 2x longer. These batteries also have a very low self-discharge rate, so they last longer when not in active use.

2. Generates More Starts

AGM batteries have the power to start a car engine more than 60,000 times. That’s about 3x more than what traditional batteries can do. 

3. Lighter and More Durable 

The AGM separator isn’t completely saturated with electrolyte, and the liquid doesn’t expand the way they do in a traditional lead acid battery. 

Less electrolyte (compared to a traditional flooded battery) means decreased weight.
No liquid expansion means the AGM battery can also withstand freezing

And while you likely won’t get any power from a frozen battery, it won’t crack or damage the plates either.

4. Low Internal Resistance Means Higher Power Output

The AGM battery has a very low internal resistance, allowing it to deliver power quickly. This is an essential function of a car battery, which pushes rapid bursts of power to start the engine.

5. Faster Recharge and Better Depth of Discharge

AGM batteries have excellent charge acceptance, which can extend their cycle life. The AGM battery can charge up to 5x faster compared to traditional batteries.

They also have an 80% Depth of Discharge (DoD) — meaning they can discharge down to 80% of their original capacity without damage. A conventional lead acid battery can typically discharge down to only 50% DoD. 

The AGM battery’s deep discharge capability also allows it to adapt well to a deep cycle application. As a result, AGM technology is often applied to the deep cycle battery format. You’ll find the deep cycle AGM battery in marine vehicles or UPS backup systems.

6. Vibration and Shock Resistant

The electrolyte-soaked glass mats in between the lead plates act like a damper. As the plates are packed fairly tightly, movement and vibrations are reduced to almost zero. This creates a battery that’s highly resistant to vibration and shock. 

7. Non-Spillable and Maintenance-Free

The AGM battery is maintenance-free and doesn’t require any watering service like a traditional wet cell battery. The absorbent glass mat between the plates holds the electrolyte in place, keeping it from spilling even when the battery is in odd positions. This allows for greater mounting flexibility.

Additionally, because the AGM battery is spill-proof, regulations are generally more relaxed about transporting them by air or road. That said, it’s best to follow the handling instructions given in the battery safety data sheets. 

Now that we’ve looked at the advantages, let’s go over the drawbacks.

2 AGM Battery Disadvantages

Here are two common drawbacks of using AGM batteries:

1. Sensitive to Overcharging

The AGM battery has a lower tolerance to overcharging and high voltages when compared to a flooded cell battery. 

2. More Costly

AGM batteries are more expensive than traditional batteries as they cost more to manufacture. On average, a conventional battery costs $65-$130, but an AGM can be over $200.

Now that you know what AGM batteries are, including their pros and cons, let’s go over some FAQs.

10 AGM Battery FAQs

Here are some answers to commonly-asked AGM battery questions:

1. Are AGM and Gel Batteries the Same?

The AGM and the gel battery are often mistaken to be the same because they’re both “dry cell” lead acid batteries. 

And while the gel cell is also a VRLA battery, it holds its electrolyte solution very differently.

Where the AGM battery uses an absorbent glass mat, the gel cell battery uses a chemical agent (like silica) to suspend the electrolyte in a gel form. The gel restricts movement, so the battery becomes spill-proof. 

Gel batteries don’t do as well as the AGM as a starter battery, so you’re less likely to find them performing that function in cars.

2. Are AGM Batteries Deep Cycle?

AGM technology is used in both deep cycle and starter battery application.

“Deep cycle” is defined by plate thickness and not the battery technology, so an AGM deep cycle battery is used as often as a flooded or gel cell deep cycle battery.

3. What Are Flooded and Sealed Lead Acid Batteries?

In a flooded lead acid battery (FLA), the lead plates are suspended in a free-flowing liquid electrolyte. It’s a wet cell battery, meaning the battery can spill and requires regular electrolyte maintenance.  

A conventional flooded battery often refers to a flooded lead acid battery.

The sealed lead acid battery (SLA battery) applies similar chemistry. But unlike a traditional flooded cell battery, the electrolyte in an SLA battery is suspended in a gel form (for gel cell batteries) or held by a glass mat (for AGM batteries). 

4. How Is a Lithium Battery Different from AGM?

AGM and lithium batteries have their own benefits and drawbacks.

The lithium ion battery is much lighter, has a better cycle life, and can charge faster than the AGM battery. Lithium ion batteries also have a flat discharge curve (meaning if you power a torchlight with a lithium battery, the bulb won’t dim as battery power runs out, it’ll just go off).

However, AGM batteries are cheaper to produce, have a higher Cold Cranking Amps (CCA) rating, and are resistant to vibration. 

It’s important to know that you can’t just swap the AGM starter battery in your car with a lithium ion battery, as your charging system probably isn’t set up to charge a lithium battery. 

Tip: Always consult a mechanic on what battery type to use if you’re unsure.

5. Can I Charge an AGM Battery with a Regular Battery Charger?

No. 

AGM batteries are sensitive to overcharging, which can affect their battery life. So, a regulated battery charger or battery maintainer is a key battery accessory you should use. 

The right AGM battery maintainer typically has microprocessors that adjust the current and voltage delivered to the battery to avoid overcharging. 

6. Do AGM Batteries Work with Solar Panels?

Yes. 

An AGM battery can be used as a solar battery. You can use a solar panel to charge one, and they’re well-suited for applications with low energy demands.

However, keep in mind that:

AGM batteries have a relatively low upfront cost compared to lithium ion batteries, which are better for high energy draws in residential solar panels.

7. What Is a Start-Stop Vehicle?

Start-stop vehicle technology automatically turns off the engine when the car stops (like at traffic lights or in stop-and-go traffic). 

While the engine is temporarily off, the car battery is the sole source of power to all the vehicle’s electrical devices, from stereo to GPS navigation. When the clutch is depressed or the brake pedal is released, it restarts the vehicle quickly and quietly. 

AGM batteries are well suited for such demanding applications.

8. What Is Battery Internal Resistance?

Internal resistance denotes a battery’s ability to deliver high currents without a significant voltage drop. 

Any current that doesn’t go into the charging translates into heat, which is why batteries get warm during heavy charging. In extreme cases, a thermal runaway can occur. 

New flooded lead acid batteries typically have 10-15% internal resistance, while a gel battery has around 12-16%. AGM batteries have among the lowest internal resistance in commercial batteries, with some as low as 2% in new batteries.

9. What’s Thermal Runaway?

It’s when too much heat is generated in a battery that can’t be expelled accordingly. If this situation continues, temperatures will increase until the battery cells dry out and the container softens and melts. 

Destruction from a thermal runaway can release toxic chemicals and cause electrical fires or batteries to explode. 

Thermal runaway occurs most often in VRLA batteries.  

10. Which is better, AGM or EFB battery?

Both EFB and AGM batteries are advanced batteries. 

An EFB battery (Enhanced Flooded Battery) is ideal for small- or mid-range vehicles with start-stop technology, while AGMs are best suited for deep cycle applications and high-powered, heavy duty vehicles like electric cars and trucks.

Final Thoughts

As vehicles evolved and developed greater power demands, battery technology had to evolve to meet these power needs. And when it comes to meeting advanced power needs, the AGM battery is currently the best one among all lead acid options.

If you are facing any issues with your existing flooded lead acid or AGM battery or have any other battery concerns, you can always look to AutoNation Mobile Service for help. 

Just contact us, and our expert technicians will be in your driveway to lend a hand!