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  • A flood lead acid (FLA) battery has positive and negative plates made of lead that are submerged in a solution of sulfuric acid and water.
  • An absorbent glass mat (AGM) battery also has lead plates suspended in an electrolyte solution but with fiberglass mats between the plates.
  • AGM batteries are spill-proof, need less maintenance, and charge faster than FLA batteries.

The car battery supplies the initial charge that kickstarts a vehicle’s electrical system. It also powers various electrical equipment, including the starter motor that cranks the engine when you turn the ignition key.

Many automotive batteries use the tried-and-tested flooded lead acid design. However, absorbent glass mat (AGM) batteries have become more popular in recent years. An AGM battery shares many features with its predecessor but outperforms lead acid batteries in many ways. But it also has drawbacks. 

It’s important to know all the details to make an informed decision on replacing your car’s standard battery with an AGM battery

What Is an FLA Battery?

The flood lead acid (FLA) battery features positive and negative plates made of lead that are submerged in a liquid electrolyte solution with acidic pH. The solution is a mix of sulfuric acid and water.

The flooded lead acid battery relies on chemical reactions to store and release electricity. These reactions create gases, which the battery vents. The battery also loses some water during the process. 

Every now and then, the battery must replenish water to keep the plates submerged and protect them from exposure to the air.If the process of how a car battery works interests you, we have an article that covers it in detail. 

What Is an AGM Battery?

The AGM battery also uses a setup with plates suspended in an electrolyte solution. But it gets its name from the fiberglass mats between its plates. These absorbent glass mats stop the solution from flowing freely by drawing off and suspending the electrolytes through adsorption.

Unlike the FLA battery, the AGM battery is fully sealed. It will only vent its gaseous byproducts in certain scenarios, such as when it’s overcharged and needs to keep internal pressure to a certain level. Otherwise, it recombines the gases with the electrolytes in the solution, effectively preventing water loss and eliminating the need to refill the battery with water.

See also  How to Tell Positive and Negative Terminals on a Car Battery

AGM batteries also need a dedicated battery charger. They cannot use the regular charger for standard batteries.

AGM vs. Standard Battery

While many features are shared between the lead acid battery and the AGM battery, they also differ in various ways. These differences can determine which battery type suits your needs and budget.

Standard and AGM Car Battery
While many features are shared between the lead acid battery and the AGM battery, they also differ in various ways.

Maintenance Needs

Drivers who are often pressed for time will appreciate the AGM battery’s lower maintenance requirements. This battery rarely needs electrolyte replenishment because it doesn’t need to vent gas except for emergencies. Venting is the leading cause of electrolyte loss.

The AGM battery can also fit in a wider number of locations because it rarely vents gas. Drivers can store it in enclosed places like the rear trunk or underneath the seat. 


Do you put your vehicle through intense use, such as off-roading or racing? Vigorous driving can damage the lead acid battery’s plates and other internal parts. The battery requires secure mounting to keep its internal parts from getting damaged. 

If you’re looking for a sturdier battery, switch to an AGM battery. Originally developed for use in aircraft and military vehicles, it’s designed with ruggedness in mind. So its sandwiched mats and plates can handle shocks much better than a standard battery. It’s also more resistant to vibration.

Mounting and Spillage

The flooded lead acid battery must always be upright. Otherwise, it can spill electrolytes that will corrode any metal part that the fluid comes into contact with.

In contrast, the AGM battery is spill proof because it’s sealed and its fiberglass mat construction lets it operate in most positions.

However, AGM batteries are smaller than flooded lead acid batteries. They might not fit in the existing battery hold downs, forcing you to buy new, compatible hold downs.

You might need to get new suitable battery hold downs if you’re switching to AGM batteries, as these batteries are often smaller than the original batteries.

Anthony Harlin, ASE Certified Master Automobile Technician

Internal Resistance and Power Output 

Resistance affects the efficiency of a battery. The lower the internal resistance, the better its voltage output and the less heat it produces. A battery that runs cool works more efficiently.

Most flooded lead acid batteries have an internal resistance of 10-15% when they’re brand new. Meanwhile, an AGM battery can have as low as 2% resistance, making it very efficient.

Another resistance-related point that the AGM battery excels in is loading, the ability to handle power demands. Among the various lead acid batteries, including the flooded and gel batteries, the one with the highest loading rating is the absorbent glass mat battery.

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Charging Time

The battery’s internal resistance also determines how fast it charges. Because the AGM battery enjoys the lowest resistance rating, it charges much faster than a standard battery. It’s not as fast as a lithium battery, but it’s a lot more accessible.

Depth Of Discharge

There’s only so much capacity that a battery can discharge before it incurs damage. This is called depth of discharge (DOD).

Typical flooded lead acid batteries have a 50% DOD rate, while AGM batteries are rated for 80% DOD. They are excellent deep cycle batteries that can steadily deliver power over a long period.

While an AGM battery can discharge much, it’s not recommended to discharge it past the 50% mark. 

Temperature Tolerance

You drive your vehicle in all kinds of weather and climate, so its battery must tolerate a wide range of temperatures, from snowy or icy winter to blistering summer. 

car battery with high temperature tolerance
You drive your vehicle in all kinds of weather and climate, so its battery must tolerate a wide range of temperatures.

When it comes to operating in all temperatures, the AGM battery beats out the standard type. It works particularly better in areas with cold weather.

The electrolyte fluid inside a flooded lead acid battery can freeze during very cold weather conditions. Like most fluids, frozen fluid will expand and take up more space. It can lead to problems like fractured lead battery plates and leaks in the battery case.

That won’t happen to an AGM battery. Its fiberglass mats prevent the electrolyte between them from expanding when it’s really cold, so there’s no chance of cold weather damage.

The AGM battery also performs better in hot weather. Its sealed design prevents evaporated electrolytes from escaping into the atmosphere. In contrast, a standard battery loses fluid when it gets too hot, whether it’s from the surroundings or from its operation.


The AGM battery is more sensitive to overcharging than a standard battery. Its normally advantageous sealed construction makes thermal runaway more likely.

When the AGM battery gets too hot and cannot cool off fast enough, it undergoes thermal runaway. Not only will it lose electrolyte fluid through evaporation, but its parts can also melt and release toxic fumes. There is also a chance that the overheating battery can catch fire or even explode. Overcharging also reduces the AGM battery’s service life.

A flooded lead acid battery is better at handling the occasional overcharge. However, it’s still not a good idea to charge it too much. For more information on how to test the car battery, read our guide.

Lifespan And Self-Discharge

AGM batteries have a longer service life than standard batteries. Their low discharge rate lets them last as long as seven years if they receive good care. In contrast, a typical flooded lead acid battery is usually only good for three to five years.

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Corrosion And Sulfation

Sulfur buildup is a problem for both AGM and standard batteries that have been left discharged for extended periods. The absorbent glass mat battery does slightly better against sulfation because it discharges at a slower rate.

However, the AGM battery is less likely to develop corrosion than other lead acid batteries. It doesn’t release evaporated electrolyte fluid very often. The likelihood of spilling or leaking electrolytes is also reduced by the sealed construction.

If the battery’s terminals have corroded or sulfated, it’s time to replace the device. Knowing how to replace a car battery can make your task easier.


The main drawback to an AGM battery is its sizable price tag. You can buy two or three standard flooded lead acid batteries for the cost of one AGM unit.

However, you do get what you pay for. An AGM battery is a big initial investment, but it will more than pay for itself over its lifetime.


In general, an AGM battery is an excellent long-term investment for your car. So if you’re interested, get one as soon as your budget can afford it.

Where to Get a Battery Replacement for Your Vehicle

Your car battery is the heart of your vehicle. So it’s best to replace it right away once you confirm that it’s malfunctioning. Good thing has a wide array of top-notch battery replacements for your ride.

All batteries sold on our site are sourced from reputable aftermarket brands, so you can be sure they’re built to last. They’ve passed strict quality tests to ensure their performance and longevity.

To get your hands on a new battery, all you have to do is go to our website and input your ride’s specific details into our built-in vehicle selector. Browse through our selection of compatible batteries, and filter the results according to your preferred brand or price range.

We want to make sure that you have a hassle-free shopping experience. If you have queries about your order, don’t hesitate to call us on our toll-free hotlines. Our team will be waiting to answer your queries.

Get back on the road in no time by getting a new battery from Shop and order today!

About The Authors
Written By Automotive and Tech Writers

The Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Contact Center Manager and Technical Reviewer at

William “Bill” Guzenski has produced hundreds of how-to videos for the automotive community. He’s an ASE Certified Master Automobile Technician, and is affiliated with the Specialty Equipment Market Association (SEMA). He loves attending race events and car shows throughout the country, as well as traveling in his 40-foot motorhome, exploring abandoned mines and ghost towns.

Reviewed By Technical Reviewer at

Tony Harlin is a Master Gas and Diesel Diagnostic Technician with over 18 years of experience. He works full-time at a large independent automotive shop as a driveability and repair technician working on all types of vehicles with a focus on diesels. ASE certifications include A1-A9, L1 and L2, as well as X1.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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