What is AGM on a Battery Charger?


Car batteries come in several different designs and types, such as valve regulated lead acid, lithium ion, and wet cell just to name a few.

As such, many newer car battery chargers have multiple modes to accommodate the various battery types.


What is AGM on a Battery Charger?

AGM is the mode on a battery charger used when charging Absorbed Glass Mat (AGM) batteries. AGM batteries are an advanced type of lead-acid battery.

Most traditional or regular battery chargers are not suited for AGM batteries. These batteries need to be charged slowly at low voltages (“slow and low”). Appropriate chargers have microprocessors that collect information from the battery and adjust the current and voltage as needed.


How Does an AGM Battery Work?

Diehard AGM batteryLead-acid batteries contain an electrolyte solution which causes a chemical reaction and produces electrons.

In simple terms, car batteries convert chemical energy to electrical energy which can, in turn, be used for things like starting a car or powering headlights.

AGM batteries have a fiberglass mat between the battery plates that absorbs the electrolyte in a dry form. The battery plates hold the battery acid.

When needed, the electrolyte is transferred from the glass mat to the plates to cause a chemical reaction and create electrons or electricity. 


What Are the Advantages of an AGM Battery?

Because of their superior construction and sealed design, AGM batteries offer several advantages:

  • Maintenance Free. Does not require water service over the life of the battery.
  • Long Lasting. AGM batteries typically have a shelf life that is 2x – 3x longer than traditional flooded batteries. 
  • Vibration Resistant. Electrolytes are absorbed into the fiberglass mat (thus the name absorbed glass mat).  Regular batteries simply hang lead plates in the electrolyte solution, and as a result, are very sensitive to external vibrations.
  • Cold Hardy. Cold definitely affects all batteries, but AGM batteries can operate in temperatures as low as minus 40 degrees Fahrenheit. 
  • No Spill Hazard. The battery case is actually fully sealed so there is no spillage when tipped. This is especially helpful with cars as battery acid can ruin car paint.
  • Lightweight. Can be up to 50% lighter than regular lead acid batteries.

AGM batteries are becoming more common in cars, especially those with stop-start fuel saving features. Historically they have been used more in RVs, boats, motorcycles and numerous cold weather applications.


What Are the Disadvantages of an AGM Battery?

Well, for starters they are priced at a 40% to 100% premium to traditional lead-acid batteries. That’s a pretty big con.

They are also highly sensitive to overcharging and undercharging. Doing either can affect both battery life and performance. See video below for details on charging AGM batteries. 

Ideally, an AGM battery should discharge no more than 50% of battery capacity. Other batteries can “deep cycle” up to 80%.

Lastly, AGM batteries don’t fare as well in hot conditions and are often installed away from engines. Stop charging if the internal battery temperature exceeds 120 degrees Fahrenheit.



Depending on your needs, AGM batteries offer many advantages that regular lead-acid batteries simply cannot match. We listed several important ones earlier.

Just make sure you know the “quirks” (e.g., must be careful when re-charging) and that your needs justify the hefty premium required to purchase an AGM battery.


Frequently Asked Questions (FAQs)

Are AGM batteries used on cars?

Most cars use traditional lead-acid batteries, but AGM batteries are becoming standard equipment on more models. They are especially suited for models with lots of electronics, stop-start fuel saving features and multiple outlets for electronic devices.

For those that don’t use their vehicles for long periods of time, AGM batteries hold their charge much better.

What are the advantages of regular lead-acid batteries?

They are much less expensive, less finicky when recharging, and in certain circumstances, are more heat tolerant.

What do you do if your battery charger doesn’t explicitly have an AGM mode?

If it doesn’t, then it is always better to err on the side of caution. You can use a trickle charger with AGM batteries if you have one. Otherwise, simply wait until you can procure the right type of charger.

Can AGM batteries be tested the same way as regular car batteries?

The answer is “no.” They have lower internal resistance than traditional batteries. As a result, most older battery testers cannot get an accurate reading. The moral of the story is: if your battery tester does not have an AGM mode, you cannot trust the readings. 

How do you prolong battery life in cold weather when a vehicle is not being used for long periods of time?

There are two common approaches. First, simply disconnect the battery cables. The battery cannot drain if it is not connected. Secondly, use a specially designed charger to “maintain” the battery during the cold months. 

Is overcharging worse than undercharging?

It is not that difficult to charge to the correct levels. That said, overcharging is a much bigger problem with AGM batteries. Remember these are low resistance batteries. Circuits are very sensitive to surges, which can easily cause permanent damage.

Is it worth it to buy a multi-purpose battery charger?

Obviously it depends on your specific needs and budget. That said, battery chargers are not overly expensive in the grand scheme of things. Generally, we prefer to buy multi-purpose chargers as a way of “future proofing” any vehicle-related needs we might have.

What is AGM on a battery charger?

Just to summarize, it is the mode on a battery charger used to re-charge Absorbed Glass Mat (AGM) batteries. Most regular chargers will not work, so make sure that your battery charger EXPLICITLY states it is designed for AGM batteries.

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