AGM Batteries: Pros and Cons

An AGM (absorbed glass mat) battery is a sealed lead acid battery that holds its electrolyte in thin fiberglass mats pressed against the lead plates, instead of letting it slosh freely like a traditional flooded battery. That design makes AGM batteries spill-proof, maintenance-free, vibration-resistant and mountable in almost any position. The trade-offs are a higher price than flooded batteries and less tolerance for overcharging and heat. For UPS systems, alarm panels, mobility scooters, wheelchairs, emergency lighting and most small powersport equipment, AGM is the standard choice, and it is what nearly every sealed lead acid battery we sell is built on.

In This Guide

• What Is an AGM Battery?
• How Does AGM Compare to Flooded Lead Acid?
• What Is the Difference Between AGM and Gel Batteries?
• What Are the Advantages of AGM Batteries?
• What Are the Disadvantages of AGM Batteries?
• How Long Do AGM Batteries Last?
• How Should You Charge an AGM Battery?
• Where Are AGM Batteries Used?
• Is an AGM Battery Right for You?
• Frequently Asked Questions

What Is an AGM Battery?

AGM stands for absorbed glass mat. Inside the battery, each lead plate is wrapped in a mat of very fine glass fibers. The sulfuric acid electrolyte soaks into those mats the way water soaks into a sponge, so there is no free liquid in the case. The battery is sealed and fitted with small pressure relief valves, which is why AGM batteries are also called VRLA (valve regulated lead acid) batteries.

The sealed design also lets the battery recombine most of the oxygen and hydrogen produced during charging back into water. A flooded battery vents that gas and slowly loses water, which is why it needs topping off. An AGM battery keeps its water, so it never needs refilling.

AGM technology was developed in the 1970s and came into wide use in the 1980s, first in military aircraft and other settings where a battery had to survive vibration, tilting and rough handling without leaking. It is now one of the two types of sealed lead acid battery. The other is gel, covered below. For the full family tree, including wet cell and maintenance-free flooded batteries, see Types of Lead Acid Batteries.

You will sometimes see AGM batteries called "dry cell" batteries. That is a loose term. The electrolyte is still liquid, it is just fully absorbed into the mats.

How Does AGM Compare to Flooded Lead Acid?

A flooded (wet cell) battery has plates submerged in liquid electrolyte, usually with removable caps for adding water. Here is how the two compare on the points that matter most when choosing a replacement.

The short version: flooded batteries cost less up front and tolerate rough charging better, but they need upkeep, ventilation and an upright mounting spot. AGM batteries cost more but can go almost anywhere and be left alone. That is why sealed equipment like UPS units, alarm panels and mobility scooters is designed around AGM.

What Is the Difference Between AGM and Gel Batteries?

Both are sealed lead acid batteries, and both are spill-proof and maintenance-free. The difference is how the electrolyte is held. AGM absorbs it into glass mats, while a gel battery mixes it with silica to form a thick paste.

• AGM has lower internal resistance, so it delivers high bursts of current well and recharges faster. It is the more common and less expensive of the two.
• Gel handles repeated deep discharges and high ambient heat somewhat better, but it delivers less peak current and needs a lower charging voltage. Charging a gel battery on an AGM profile can damage it.

Because the charging profiles differ, replace gel with gel and AGM with AGM unless the equipment maker says otherwise. Gel batteries are clearly marked "GEL" on the label. Lithium is a separate chemistry entirely. We compare it to sealed lead acid in SLA Batteries vs Lithium Batteries.

What Are the Advantages of AGM Batteries?

• Spill-proof. With no free liquid inside, a cracked AGM battery will not pour acid. That makes it safer to handle, install and transport.
• Mounts in almost any orientation. AGM batteries can sit upright, on their side or on their end, which gives equipment designers room to fit them into tight compartments. Most manufacturers advise against mounting them fully upside down.
• Maintenance-free. No water to check or add, and no acid to measure. Install it and leave it alone.
• Strong vibration and shock resistance. The tightly packed mats hold the plates firmly in place. That is why AGM was adopted in aircraft and is now standard in motorcycles, ATVs, personal watercraft and other equipment that rattles and bounces.
• Very little gassing. Most of the gas produced during charging recombines inside the battery, so AGM batteries can be used in enclosed cabinets and indoor equipment where a flooded battery would need ventilation.
• Low self-discharge. A healthy AGM battery loses roughly 3% of its charge per month at room temperature, so seasonal and standby equipment holds its charge far longer between top-ups.
• Good cold-weather performance. AGM batteries deliver power well in the cold. A fully charged AGM battery is also very resistant to freezing, although a deeply discharged one can still freeze. Keeping it charged is the best protection.
• High current on demand. Low internal resistance lets AGM batteries supply strong bursts of current. That suits engine starting and UPS systems that must carry a full load the instant the power fails.
• Simpler shipping. Batteries that meet the U.S. DOT non-spillable requirements (49 CFR 173.159a) are exempt from many of the hazardous materials rules that apply to flooded batteries.

What Are the Disadvantages of AGM Batteries?

• Higher purchase price. The glass mat separators and sealed construction cost more to manufacture, so an AGM battery typically costs more than a flooded battery of the same size.
• Sensitive to overcharging. Because an AGM battery is sealed, any water it loses to overcharging is gone for good. Sustained high voltage dries the mats out and shortens the battery's life. Use a charger with the correct AGM or SLA profile.
• Sensitive to heat. Heat is the biggest life-shortener for any sealed lead acid battery. As a common rule of thumb, service life roughly halves for every 15 to 18°F (8 to 10°C) the battery operates above about 77°F (25°C). A UPS in a hot closet will need new batteries much sooner than one in a cool room.
• Shorter life under repeated deep discharge. Draining an AGM battery completely over and over wears it out far faster than shallow discharges do. Equipment that routinely runs the battery flat will go through batteries quickly.
• Should be recharged promptly. An AGM battery left partly discharged for long periods develops sulfation, which permanently reduces capacity. See SLA Battery Sulfation.
• Heavy compared with lithium. AGM weighs about the same as a flooded battery of similar capacity, but much more than a lithium battery with the same usable energy.
• Cannot be tested with a hydrometer. The sealed case means you cannot measure specific gravity the way you can with a flooded battery. Use a voltmeter or a conductance tester instead. See SLA Battery Testing.

How Long Do AGM Batteries Last?

In standby use, where the battery sits on a float charge waiting for a power outage, a standard AGM battery typically lasts 3 to 5 years. Long-life series designed for telecom and data center use are rated for 10 years or more. That is the life you can expect in UPS systems, alarm panels and emergency lighting kept at normal room temperature.

In cyclic use, where the battery is drained and recharged regularly, life is measured in charge cycles, and depth of discharge is what matters most. An AGM battery that is regularly drained only partway will deliver many more cycles than one that is run flat every time. Mobility scooters, wheelchairs and ride-on toys fall into this category.

Three things shorten AGM life more than anything else:

1. Heat. Keep batteries as close to room temperature as the equipment allows.
2. Deep discharges. Recharge before the battery is fully drained whenever you can.
3. Wrong charging. Overcharging dries the battery out, and chronic undercharging sulfates it.

For day-to-day habits that extend battery life, see SLA Battery Care.

How Should You Charge an AGM Battery?

Use a charger designed for AGM or sealed lead acid batteries. A good AGM charger works in stages: a bulk stage that delivers most of the charge, an absorption stage that holds a set voltage until the battery is nearly full, and a float stage that keeps it topped off without overcharging.

For a typical 12V AGM battery, published charging values fall roughly in these ranges at room temperature:

• Cyclic charging (absorption): about 14.4 to 14.9 volts
• Standby or float charging: about 13.5 to 13.8 volts

Always check the label on your battery. Most AGM batteries print their exact cycle and standby voltages and their maximum initial charging current right on the case. Older flooded-battery chargers without an AGM setting can overcharge an AGM battery, and gel settings can undercharge one. For step-by-step guidance, see Charging SLA Batteries.

Where Are AGM Batteries Used?

AGM is the default battery for sealed equipment that has to work reliably without maintenance. Common uses include:

• UPS and battery backup systems: UPS batteries for computers, servers and network equipment
• Alarm and security panels: alarm batteries, usually small 12V 4Ah to 7Ah units
• Emergency and exit lighting: emergency light batteries
• Mobility equipment: mobility scooter batteries and wheelchair batteries
• Medical devices: medical equipment batteries
• Motorcycles and powersports: motorcycle batteries and powersports batteries for ATVs, UTVs and personal watercraft
• Kids' ride-on toys: ride-on toy batteries
• Fish finders and small marine electronics: fish finder batteries
• Solar and off-grid storage: solar batteries

Many of these systems run two or more 12V AGM batteries wired in series through a single charger. A series pack performs only as well as its weakest battery, so when one battery fails, replace the whole set with matching batteries of the same capacity and age. Mixing a new battery with old ones shortens the life of the new one. See Series and Parallel Connections for how these packs are wired.

AGM batteries are also common in modern cars, especially vehicles with automatic start-stop systems, which put heavier demands on the starting battery. BatteryClerk does not carry automotive starting batteries. For a car, check with your vehicle dealer or an auto parts retailer, and match the battery type your vehicle came with.

Is an AGM Battery Right for You?

An AGM battery is the better choice when:

• the battery sits inside a sealed device or cabinet, or indoors
• it might be tipped, mounted on its side or exposed to vibration
• it spends long periods on standby or in seasonal storage
• nobody will be checking water levels
• the original battery was AGM, which covers almost all UPS, alarm and mobility equipment

A flooded battery may make more sense when cost is the main concern, the battery sits upright in a ventilated space and someone will maintain it. If your equipment was designed for lithium, or you are weighing a switch to lithium, read our SLA vs lithium comparison first. The charger and the equipment both have to support the change.

Frequently Asked Questions

What does AGM stand for on a battery?

AGM stands for absorbed glass mat. It describes how the battery holds its electrolyte: soaked into thin fiberglass mats between the lead plates instead of as free liquid. AGM batteries are a type of sealed lead acid (SLA) battery, also called valve regulated lead acid (VRLA).

Is an AGM battery the same as a sealed lead acid battery?

AGM is one of the two types of sealed lead acid battery. The other is gel. Most sealed lead acid batteries sold for UPS systems, alarms, mobility scooters and powersports equipment are AGM, so the two terms are often used interchangeably.

How do I know if my battery is AGM?

Check the label for "AGM," "VRLA," "sealed lead acid" or "non-spillable." An AGM battery has no removable caps for adding water. Gel batteries are labeled "GEL." If the label is worn off, the equipment manual or the original part number will identify the battery type.

Can I use a regular battery charger on an AGM battery?

Only if the charger has an AGM or sealed lead acid setting, or is rated for AGM batteries. Older chargers built for flooded batteries can push too much voltage into an AGM battery, drying it out and shortening its life. A multi-stage smart charger with an AGM profile is the safest choice.

Can an AGM battery be mounted on its side?

Yes. Because the electrolyte is fully absorbed into the glass mats, an AGM battery can be mounted upright, on its side or on its end without leaking. Most manufacturers recommend against mounting it fully upside down.

Can AGM batteries freeze?

A fully charged AGM battery is very resistant to freezing. A deeply discharged one can freeze, because the electrolyte becomes closer to plain water as the battery discharges. Keep AGM batteries charged during cold-weather storage to protect them.

Are AGM batteries worth the extra cost?

For most sealed equipment, yes. AGM batteries need no maintenance, will not spill, tolerate vibration and hold their charge in storage. In UPS systems, alarms, mobility scooters and powersports equipment, AGM is usually the only type that fits and works safely. A flooded battery only saves money where it can sit upright, be ventilated and be maintained.

Should I replace all the batteries in a pack at the same time?

Yes, if the batteries are wired together in series, as they are in most mobility scooters, wheelchairs and larger UPS units. A series pack only performs as well as its weakest battery, so a new battery paired with worn ones will be dragged down and fail early. Replace them as a matched set.

Find Your Replacement AGM Battery

Search by your equipment's brand and model, or browse our full range of sealed lead acid batteries. Popular categories include UPS batteries, alarm batteries, mobility scooter batteries and powersports batteries. When your old battery reaches the end of its life, see Recycling SLA Batteries for how to dispose of it safely.