Group 31 Battery: Size, Terminals, Chemistry, and How to Pick One
A group 31 battery is one of the largest 12-volt sizes you’ll find in a pickup, a bass boat, or an RV battery compartment. The label tells you how big the case is and where the posts sit. It doesn’t tell you whether the battery can start an engine, survive deep discharges, or handle cold mornings. Those answers depend on chemistry and design. This guide walks through the official size limits, terminal styles, flooded, AGM, and lithium options, and where a separate power station fits when you only need AC outlets at camp.
What is a Group 31 battery?
"Group 31" is a size code from Battery Council International (BCI). BCI is the trade group behind the numbering system US battery makers use. A group number sets the maximum case size and the terminal layout. That way, a replacement fits the tray and lines up with the cables. It is not a capacity rating.
BCI sorts batteries by voltage, maximum dimensions, terminal layout, and features that affect fit. Batteries in one group must match in size within 2 mm. BCI assumes flooded lead chemistry unless noted. AGM, EFB, and lithium (such as "LFP") are marked separately.
BCI’s group chart lists group 31 under Heavy-Duty Commercial Batteries, 12-volt, as 31A and 31T. Both top out at 13 x 6 13/16 x 9 7/16 inches (330 x 173 x 240 mm). The terminals differ. The 31A uses Type A, a top SAE post. The 31T uses Type T, a threaded post. Both also list marine Types M and M1. BCI notes the designs are for reference only.
You will also see "31M." Makers add the M for marine versions. Interstate lists its SRM-31 as "Group Size 31M," and ODYSSEY sells the ODX-AGM31M. Treat the M as a maker label for a marine terminal style. Check the exact terminal type on the spec sheet.
Group 31 battery size compared with groups 24 and 27
BCI group | Max length | Max width | Max height | BCI section |
24 | 10 1/4 in | 6 13/16 in | 8 7/8 in | Passenger Car and Light Commercial |
27 | 12 1/16 in | 6 13/16 in | 8 7/8 in | Passenger Car and Light Commercial |
31 (31A, 31T) | 13 in | 6 13/16 in | 9 7/16 in | Heavy-Duty Commercial |
Groups 24, 27, and 31 share the same width. Group 31 is longer and taller. It won’t drop into a tray or box built for a 24 or 27. Check clearance and hold-down fit first.
Real batteries run a little under the BCI maximums, and they vary by maker. The RELiON RB100 lists 13.0 x 6.8 x 8.4 inches. The Interstate SRM-31 lists 13 x 6 3/4 x 9 5/8 inches. That height is above BCI’s 9 7/16-inch maximum. So measure your tray. Then compare it with the spec sheet for your battery, including height with terminals.
Group numbers describe the case, so capacity has to come from the amp-hour rating. Two group 31 batteries can look identical, so knowing what amp hours really measure helps you compare them.
Size gets the battery into the tray, but what’s inside the case decides weight, lifespan, and how you charge it.
Group 31 battery types: flooded, AGM, and lithium (LiFePO4)
Group 31 batteries come in three common chemistries. They share a footprint but behave very differently. Flooded lead-acid is the BCI default. AGM is a sealed lead-acid design. Lithium iron phosphate (LiFePO4) is the lighter option. Each one has its own charging needs.
Example (maker spec page) | Chemistry | Rated capacity | Weight | Terminals | Listed use |
Interstate SRM-31 | Flooded | 98 Ah; 210 min reserve capacity at 25A; 675 CCA | Not listed | Top | Deep cycle (marine/RV) |
ODYSSEY ODX-AGM31M (31M-PC2150) | AGM | 103 Ah (20-hr); 220 min RC; 1,150 CCA | 77.8 lb | Brass SAE posts plus stainless threaded studs | Extreme Series marine |
RELiON RB100 | LiFePO4 | 100 Ah, 12.8V, 1,280Wh | 26.4 lb | M8 x 1.25 | Deep cycle (not for engine starting) |
The table shows maker examples only. It is not a ranking.
Flooded
These need upright mounting and venting. The EPA notes that lead-acid batteries can contain up to 18 pounds of lead. They can also hold about a gallon of corrosive, lead-contaminated sulfuric acid. Handle them with care.
AGM
This is a sealed lead-acid design. It is heavier than lithium. In the table example, the AGM weighs 77.8 lb and the lithium model weighs 26.4 lb.
LiFePO4
This type is lighter, but it needs a compatible charge profile. RELiON’s own page recommends a lithium profile. It says AGM or gel profiles also work. It advises against a flooded profile, which can trigger overvoltage disconnects. For this model, the recommended charge voltage is 14.2 to 14.6V. The charge temperature range is -4 to 113°F. Cold-charging limits vary by model, so follow your battery’s sheet.
This guide skips lifespan years and cycle counts. Those numbers vary by maker and model. Check the sheet for the battery you plan to buy.
Once you see how LiFePO4 battery chemistry works, the weight gap with AGM makes sense.
Swapping chemistries inside the battery compartment means checking chargers, cables, and fit. That is more than some campers want to take on just to run a few outlets.
Getting AC power at camp without rewiring your Group 31 setup
A group 31 battery is a 12V DC battery. Running household-style AC devices from it takes an inverter, heavy cables, and fuses sized by someone who knows the system. If you only need outlets for lights, phones, or a portable fridge at camp, a separate portable power station gives you AC output. It leaves the vehicle’s battery system alone.
Here is the gap in numbers. A 100Ah, 12.8V lithium group 31, like the RELiON RB100, holds 1,280Wh nominal (maker spec). Yet it delivers DC only. A power station packs a battery, an inverter, and AC outlets into one case.
The boundary is simple. Neither EcoFlow unit is a group 31 battery or a swap for one. Neither can start an engine. The vehicle’s batteries still need a compatible charger and their own protection. Don’t wire a power station into an RV’s 12V system or a boat’s battery bank.
Placement matters too. Keep the unit inside the RV or in a dry spot. Keep it out of spray and rain, and within its listed temperature ranges.
The EcoFlow DELTA 3 line fits the gap between 12V DC and AC outlets. These units sit next to your setup instead of inside it. Each one carries its own inverter. Each recharges from solar, a wall outlet, or a generator. Two fit this job: the EcoFlow DELTA 3 Classic for lights and devices, and the EcoFlow DELTA 3 Max Plus for fridge-level loads.
A portable fridge changes the energy budget. It cycles on and off all day and night. That steady draw is where the higher-capacity unit earns its spot, since small loads add up over many hours.
Runtime estimates
Use these formulas to size a unit for your own gear. The 0.85 factor allows for inverter and conversion losses.
EcoFlow DELTA 3 Max Plus with a portable fridge: 2,048Wh × 0.85 ÷ [fridge’s average watts] = estimated hours. Find the average watts on the fridge’s spec label, or measure it with a plug-in meter.
EcoFlow DELTA 3 Classic with lights and phones: 1,024Wh × 0.85 = about 870Wh usable. Divide that by the combined watts of your actual lights and chargers.
How do the two units divide the job?
The EcoFlow DELTA 3 Classic covers lighting and communications at camp. Think about phones, lanterns, and laptops. The EcoFlow DELTA 3 Max Plus adds room for a portable fridge and other higher-energy needs. Both stay separate from the group 31 battery and the vehicle’s charging system.
If you’re comparing batteries that suit dry camping, think about AC needs separately from the house bank.
Back inside the battery compartment, the next choice is whether your group 31 has to start an engine, run accessories, or do both.
Group 31 deep cycle vs. starting vs. dual purpose
The same group 31 case can hold a battery built for very different jobs. Starting batteries deliver a short, heavy burst for an engine. Deep cycle batteries deliver steady current to house loads or a trolling motor. Dual-purpose models try to cover both.
The design differences come down to plate construction and how deeply a battery is meant to discharge. Knowing how deep cycle batteries differ from starting batteries keeps you from cranking an engine on the wrong one.
How to read the ratings?
Cold cranking amps (CCA) matter for starting. BCI’s chart measures CCA at 0°F. Amp-hours and reserve capacity matter for deep cycle use. BCI’s chart lists original-equipment CCA reference ranges. For 31A, the range is 660 to 1,000. For the 31T, it is 300 to 1,300.
Examples from maker pages
The Interstate SRM-31 is sold as a deep cycle, with 675 CCA and 210 minutes of reserve capacity.
The Interstate 31M-AGM is sold for dual purpose, with 925 CCA.
The RELiON RB100 is a deep cycle. RELiON points engine-starting users to a different model.
Marine and RV use
Use deep cycle batteries for household loads, trolling motors, and accessories. Use starting or dual-purpose batteries where the battery also cranks the engine. Match the vehicle or boat maker’s requirement.
Once the type is settled, the last details keep a heavy battery safe. They are terminals, boxes, hold-downs, and charging.
Group 31 battery boxes, terminals, and safe charging
A group 31 battery is heavy and holds a lot of energy. In lead-acid form, it also contains acid. Mounting it well, covering the terminals, and using the right charger matter as much as picking the battery. Federal boat-building rules spell out several of these basics.
What does the Coast Guard require?
The Battery Council International (BCI) sets the group sizes, so group 31 has an official definition. In the BCI Group Numbers and Dimensional Specifications table, group 31 is listed as a 12-volt heavy-duty commercial battery in two versions: 31A (top post) and 31T (threaded stud). Both have maximum overall dimensions of 13 x 6 13/16 x 9 7/16 inches (330 x 173 x 240 mm). Those are the largest sizes allowed, so always check the actual battery’s spec sheet before buying a box or tray.
The Coast Guard’s battery rule is narrower than many people think. Under 33 CFR 183.401, the electrical rules in this subpart apply only to boats with gasoline engines, other than outboard engines, used for propulsion, power, or electrical generation. The battery section, 33 CFR 183.420, is listed under “Manufacturer Requirements.” The Coast Guard reprints it in its Boatbuilder’s Handbook (USCG). For those boats, it requires builders to make sure that:
Each installed battery moves no more than one inch in any direction during a pull test. The test force is 90 pounds or twice the battery’s weight, whichever is less.
Metal objects can’t touch the ungrounded battery terminals.
The battery doesn’t sit directly above or below a fuel tank, fuel filter, or fuel line fitting.
Hydrogen gas released by the battery has a way to vent out of the boat.
These are legal requirements for builders of gasoline inboard and sterndrive boats only. They don’t cover outboard-only boats, diesel boats, or RVs. Still, they make a sensible checklist for any group 31 installation: secure it, cover the terminals, keep it away from fuel, and give it ventilation.
Conclusion
Group 31 is a BCI size code. The maximum is 13 x 6 13/16 x 9 7/16 inches, with set terminal layouts. Chemistry and design decide weight, charging, and whether the battery can start an engine.
Before you buy, measure the tray and confirm the terminal type. Match a deep cycle or starting battery to the job. Then use the charger profile the maker lists.
If you just want AC outlets at camp without touching the battery system, an EcoFlow DELTA 3 Classic or EcoFlow DELTA 3 Max Plus can handle that on its own. The former suits lights, phones, and a laptop, while the latter adds room for a portable fridge. Both run beside your setup, so your group 31 battery and its charging stay exactly as they are.
FAQs
What size is a group 31 battery?
BCI lists a maximum of 13 inches long, 6 13/16 inches wide, and 9 7/16 inches tall for groups 31A and 31T. Actual batteries often measure a bit smaller, so check the maker’s spec sheet.
What's the difference between 31A, 31T, and 31M?
The letter refers to terminals. BCI’s 31A uses SAE top posts, and 31T uses threaded posts. Makers often label marine versions with dual terminals as 31M.
Is a group 31 battery bigger than a group 27?
Yes, it’s longer and taller. BCI lists group 27 at up to 12 1/16 inches long and 8 7/8 inches tall. Group 31 is 13 inches long and 9 7/16 inches tall.
Can I replace a group 31 AGM battery with lithium?
Often the case fits, but the charging setup has to match. Lithium batteries need a charger and settings their maker approves. Some lithium models aren’t meant to start engines.
Can a portable power station replace a group 31 battery in my RV?
No. It’s a separate AC power source, not a 12V battery. An EcoFlow DELTA 3 Classic or EcoFlow DELTA 3 Max Plus can run AC devices at camp while the RV’s battery system stays as it is.
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