Group 31 Marine Batteries: Fit, Capacity and Options

June 17, 2026 · Marine Battery Source

Group 31 battery mounted beside an inboard engine in a boat's engine compartment

Group 31 is the largest of the common marine sizes before you move up to 4D and 8D. It’s popular for high-cranking starting batteries on larger outboards, dual-purpose batteries on center consoles, house batteries, and many native 24V and 36V lithium trolling batteries. This guide covers the size, ratings and best uses. Check product pages for exact dimensions and current specs.

Group 31 size

Typical BCI dimensions for a group 31 are about 13 inches long, 6.8 inches wide and 9.4 inches high. It’s roughly an inch longer and half an inch taller than a group 27. Many group 31 batteries use threaded stud terminals instead of round posts, so check your cable lugs. Marine versions are labeled 31M.

Lead-acid group 31 options

Model	Type	Capacity	CCA	Reserve	Weight
Interstate SRM-31	Flooded deep cycle	98Ah	675	210 min	Not listed
Interstate 31M-AGM	AGM dual purpose	100Ah	925	200 min	Not listed
Interstate 31M-AGM-A	AGM dual purpose	100Ah	825	200 min	Not listed
Trojan 31-AES	AGM deep cycle	104Ah	Not listed	180 min	69 lb
Trojan AES-31 (29/31)	AGM deep cycle	104Ah	Not listed	177 min	67 lb

At-a-glance summary: Group 31 is about 13 x 6.8 x 9.4 inches and often uses threaded stud terminals; Lead-acid group 31 batteries in our store list 98–104Ah and 675–925 CCA where rated; Lithium group 31 cases range from 36V 50Ah trolling batteries to 12V 334Ah house batteries — Key takeaways: Group 31 Marine Batteries: Fit, Capacity and Options

Lithium group 31 options

Model	Voltage	Capacity	Weight	Notes
EPOCH 12V 172Ah	12V	172Ah	40 lb	Dual purpose, 1,000 CCA equivalent, IP65
EPOCH 12V 300Ah Essential	12V	300Ah	58 lb	Heated, Bluetooth
EPOCH 12V 314Ah Eco	12V	314Ah	57 lb	Value line, no heating
EPOCH 12V 334Ah Essential	12V	334Ah	60 lb	Heated, Bluetooth
EPOCH 36V 50Ah Essential	36V	50Ah	35 lb	Heated, IP65
Battle Born 100Ah	12V	100Ah	31 lb	Fits group 27/31
Battle Born 24V 50Ah	24V	50Ah	31 lb	Fits group 27/31
Interstate Li31M 24V	24V	60Ah	Not listed	60-month warranty
Interstate Li31M 36V	36V	50Ah	Not listed	60-month warranty

Best uses for a group 31

High-cranking starting: larger outboards and inboards often call for 800 CCA or more. A group 31 AGM like the 31M-AGM, at 925 CCA, covers most needs. Always check your engine maker’s requirement, and remember that cranking power falls in cold weather, so northern boaters may want extra margin.

Dual purpose on center consoles: a group 31 AGM can start the engine and run electronics, pumps and lights, with 200 minutes of reserve.

House batteries: a group 31 deep-cycle battery offers about 50Ah usable in lead-acid. A group 31 lithium battery offers far more: an EPOCH 12V 300Ah gives roughly 240–300Ah usable, about what five or six lead-acid group 31s would provide.

24V and 36V trolling: single-case 24V and 36V lithium batteries in group 31 size replace two or three lead-acid batteries, saving weight and removing series jumpers.

Weight

Lead-acid group 31 batteries weigh about 67–69 pounds where listed, among the heaviest batteries most boaters lift. A group 31 lithium battery weighs 31–60 pounds depending on capacity, and the larger ones store several times the energy.

Stud terminals

Many group 31 batteries have 3/8-inch or 5/16-inch threaded studs. Use ring terminals sized for the stud, with a lock washer and nut torqued to the maker’s spec. Don’t stack too many ring terminals on one stud; use a bus bar or distribution block for multiple circuits. EPOCH sells a heavy-duty power distribution block for this.

Charging

For lead-acid group 31s, a 10A–20A charger per battery works well. AGM batteries need an AGM profile. For lithium, use a LiFePO4 profile and check the maximum charge current; EPOCH’s 12V 300Ah Essential lists 200A maximum continuous charge, so almost any marine charger is within limits. For large lithium house batteries, a higher-output charger, like EPOCH’s 12V 50A, shortens charging time.

Trays and mounting

Group 31 batteries are long and heavy, so use a sturdy tray with a strap. EPOCH sells a group 31 tray for its 12V 172Ah dual-purpose battery and trays for its 300Ah, 314Ah and 334Ah models. Check the tray’s part number against your battery.

Upgrading from group 27 to group 31

If your box allows, moving from a 27 to a 31 adds a little capacity in lead-acid. The bigger gain comes from switching chemistry: a group 31 lithium battery can hold three times or more the usable energy of the group 27 lead-acid battery it replaces. Check length and height before ordering, and confirm your cables and terminals fit.

Common group 31 setups

Center console, single outboard: a group 31 AGM start battery plus a second group 31 for house loads, joined by a battery switch or automatic charging relay.
Bass boat with a 36V trolling motor: a group 31 AGM start battery plus a single-case 36V lithium trolling battery in group 31 size.
Small cruiser: a group 31 lithium house battery with a DC-DC charger from the engine and a lead-acid or AGM start battery.

In each case, keep the start battery separate so house or trolling use can’t leave you unable to start the engine.

Warranty

Interstate’s group 31 AGM models list 24 to 36 months of free replacement; the SRM-31 lists 12 months. EPOCH Essential models list 11 years and Eco models 6 years. Trojan’s listings don’t show warranty terms, so check Trojan’s terms.

Products mentioned

Frequently asked questions

What size is a group 31 battery?

Typically about 13 x 6.8 x 9.4 inches. Check the listing for exact dimensions.

What terminals does a group 31 have?

Often threaded studs. Some have both studs and posts.

How many amp-hours is a group 31?

Lead-acid models in our store list 98–104Ah. Lithium 12V models list 100–334Ah.

Is group 31 good for starting?

Yes. Group 31 AGM batteries like the 31M-AGM list up to 925 CCA.

How heavy is a group 31 battery?

Lead-acid models weigh about 67–69 pounds where listed. Lithium models weigh 31–60 pounds.