What Is a Group 31 AGM Battery?

On this page (8 sections)

1. What does "Group 31" actually mean?
2. What makes an AGM battery different from flooded?
3. What is a Group 31 AGM battery used for?
4. How big is a Group 31 AGM battery in inches?
5. How much cranking power and capacity should you expect?
6. What should you check before buying one?
7. Which AGM batteries suit readers who don't need a Group 31?
8. Is a Group 31 AGM battery worth it?

A Group 31 AGM battery is a large 12V absorbed glass-mat battery about 13 x 6.8 x 9.4 inches, built to deliver high cranking amps and survive repeated charging and discharging. It's the size used in heavy-duty trucks, marine and RV house banks, and off-grid power setups.

If you've been told to buy "a Group 31 AGM" and you're not sure what that means, the answer comes down to two things. Group 31 is the physical size class from the BCI standards.

AGM is the internal build: lead plates wrapped in glass-fiber mats that hold the acid instead of letting it slosh.

What does "Group 31" actually mean?

Group 31 is a BCI size class, not a brand or a power rating. The Battery Council International assigns group numbers so a battery of that class drops into the same tray and clamp across brands.

The typical Group 31 case measures roughly 13 inches long, 6.8 inches wide, and 9.4 inches tall. Posts sit on top in a set pattern, so check your old battery's terminal sides before ordering.

That footprint is bigger than a Group 35 or a group 24f car battery, and heavier too. Most Group 31 AGM batteries land between 55 and 75 lb, so plan a lift strap and a second pair of hands.

A group number does not fix capacity or cranking power. Two Group 31 AGM batteries can differ widely in amp-hours and cold cranking amps, so the group size narrows the fit and the label settles the performance.

What makes an AGM battery different from flooded?

AGM batteries hold their electrolyte in glass-fiber mats, so there's no free liquid to spill. The acid is absorbed into the mat, and the battery is sealed and valve-regulated.

That build brings real trade-offs. AGM tolerates vibration better, resists acid leaks when tipped, and self-discharges more slowly than a flooded battery.

The Interstate MTX line, for example, uses a pure lead AGM design that the brand says packs more lead into the case for longer life.

The cost is higher. AGM usually costs more than a flooded battery of the same group size.

AGM also needs a charger with an AGM profile, because a flooded-battery charger can push too much voltage.

One limit matters for buyers: most Group 31 AGM batteries sold as automotive or marine starting batteries are not deep-cycle units. Check the label before you plan to run a trolling motor or a cabin off one.

What is a Group 31 AGM battery used for?

The typical uses are heavy-duty starting and deep-cycle power. The size and AGM build make it a fit for jobs where a small car battery would struggle.

Common applications include:

• Diesel pickups and work trucks, where the engine needs a big crank and the truck frame has room for the long case.
• Marine starting and house banks, from bass boats to cruisers, because AGM resists vibration and won't spill acid in a rough chop.
• RV house batteries, where AGM's low self-discharge helps a rig that sits between trips.
• Off-grid and solar storage, though only on deep-cycle-rated models.
• Commercial equipment, like lift gates and auxiliary power units.

For lighter vehicles, a smaller size is the better fit. If your car calls for a Group 35 or a group 65 battery, the 31 case likely won't clear the hold-down, and the extra weight buys you nothing.

When should you pick a Group 31 AGM over a smaller size?

Pick a Group 31 AGM when your vehicle or equipment specifies the 31 size and you want AGM's spill resistance and cycle life. It's the right call for a truck that cranks a big diesel, a boat with a house bank, or an RV that runs accessories off the battery.

Skip it when the tray is smaller. A Group 31 won't fit a Group 35 or Group 48 opening, and forcing it in risks a loose battery and a damaged hold-down.

If you're shopping for a car-sized AGM, our list of group 48 battery options covers that footprint.

How big is a Group 31 AGM battery in inches?

Plan on about 13.0 x 6.8 x 9.4 inches, though case molds vary by a fraction of an inch between brands. Measure your tray and the space above it before you buy.

The height matters as much as the footprint. A taller cap or a different terminal style can foul a battery box lid or a truck's hold-down bar.

A Group 31 is tall enough that a car's battery compartment often won't close over it.

Weight is the other dimension people forget. At 55 to 75 lb, a Group 31 AGM is heavy enough that lifting it by the posts is a bad idea.

Use the case handles if it has them.

How much cranking power and capacity should you expect?

A Group 31 AGM commonly runs from about 700 to 1,200 cold cranking amps, with 100 to 125 amp-hours of capacity on deep-cycle models. The exact figures sit on the label and vary by model.

CCA tells you how well it starts a cold engine. AH tells you how long it can run a load.

A battery with high CCA and modest AH is a starting battery; one with high AH and moderate CCA is built for cycling.

Match the CCA to your engine's requirement first. If your truck's manual asks for 800 CCA, a 700 CCA battery is under-spec, even if the case fits.

Then check AH if you plan to run accessories with the engine off.

AGM also charges faster than flooded in many cases. The Daakmax Platinum Group 35, a smaller AGM, illustrates the pattern with a high-density paste the brand says improves conductivity and recharge times.

The same AGM behavior carries over to the 31 size.

What should you check before buying one?

Check the group size, terminal layout, CCA, and whether it's a starting or deep-cycle battery. Those four points catch most buying mistakes with Group 31 AGM batteries.

Start with the old battery's label. Read the group number, the CCA, and the terminal position.

A 31T has a different post arrangement than a plain 31, so the letter matters.

Then check the charger. An AGM-compatible charger with an AGM setting keeps the battery healthy.

A basic flooded-battery charger can shorten AGM life.

Finally, confirm the application. A starting battery is for cranking; a deep-cycle battery is for running loads.

Using a starting battery in a deep-cycle role, like a trolling motor bank, wears it out early.

If you're comparing AGM options in a different footprint, our group 35 battery picks and the Interstate AGM Group 65 below show how the size classes differ in practice.

Which AGM batteries suit readers who don't need a Group 31?

If your vehicle takes a smaller group, buy the size it specifies rather than upsizing to a 31. These three AGM batteries cover the common smaller footprints and are worth comparing by fit and CCA.

Interstate AGM Group 65

The Interstate MTX-65 is a 12V 68Ah Group 65 AGM rated at 750 CCA. It's a pure lead AGM design that Interstate says delivers 2X to 3X the life of conventional batteries.

It's the pick here for a full-size truck or SUV with a Group 65 tray. The 31 size won't fit those vehicles, and the 65 case gives you a straight swap.

Daakmax Platinum AGM Group 35

The Daakmax Platinum is a 12V 55Ah Group 35 AGM with 650 CCA and a 36-month warranty. It's built for start-stop vehicles and lists a -22°F to 158°F operating range.

This one suits a sedan or small SUV that came with an AGM Group 35. It's a starting battery, so don't plan on using it for a trolling motor or solar bank.

Autocessking AGM Group 35

The Autocessking Group 35 is also a 12V 55Ah AGM at 650 CCA. It's a starting battery with a 3-year warranty, and the brand warns against deep-cycle use.

Choose it if you want a second Group 35 AGM option for a car or small truck. Like the Daakmax, it's for cranking, not for cycling a cabin's power.

Is a Group 31 AGM battery worth it?

It's worth it when the size fits and you need the cranking power or cycle life. For a diesel truck, a boat, or an RV house bank, the Group 31 AGM is the size and build that do the job.

It's not worth it as an upgrade for a car that takes a Group 35 or a Group 65. The bigger case won't fit, and the extra weight and cost buy you nothing.

So what is a Group 31 AGM battery in one line? It's the large, sealed AGM size built for heavy starting loads and repeated cycling.

Confirm your group number and terminal layout, match the CCA to the engine, and buy the size your vehicle actually specifies.

Frequently asked questions

Is a Group 31 AGM battery the same as a 31T?

No, the T marks terminal layout. A Group 31 AGM has its posts in one arrangement and a 31T puts them where a truck needs them, so match the terminal position to your old battery before buying.

Can a Group 31 AGM replace a flooded Group 31?

Yes, if your charging system supports AGM charging voltages. AGM and flooded batteries share the same footprint and post layout in most cases, but a simple old charger may not bring an AGM battery to full charge.

How long does a Group 31 AGM battery last?

Expect 4 to 6 years in a starting application and 3 to 5 years in deep-cycle service, depending on how deeply you discharge it and how well you recharge it. Shallow cycles and prompt recharging stretch life.

How much does a Group 31 AGM battery weigh?

Around 55 to 75 lb, which makes it the heaviest size most people handle. Plan two hands, a lift strap, and a helper when you install one in a truck frame or a boat locker.

Do I need a special charger for a Group 31 AGM battery?

Yes, an AGM-compatible charger set to the AGM profile. A charger meant for flooded batteries can overcharge AGM and shorten its life, and a trickle charger without an AGM setting may stop short of full charge.