31 Group Size

Group 31 spans everything from a basic flooded starter to a premium lithium bank. Same case, very different battery inside. This guide breaks down what a group 31 deep cycle battery actually is, how it stacks up against other group sizes, which type fits your boat, RV, or solar setup, and the specs worth knowing before you buy.

What Is a Group 31 Battery?

A BCI group 31 battery is built to a shared case size, not a specific chemistry or brand. The designation comes from the Battery Council International (BCI), which standardizes physical dimensions so batteries fit common boxes, trays, and mounts across manufacturers.

Group 31 battery dimensions run about 13 inches long, 6.8 inches wide, and 9.44 inches tall. That's the group 31 battery size whether it's flooded, AGM, gel, or lithium inside.

Weight is where things diverge. A lithium group 31 battery weighs around 30 pounds. A flooded or AGM lead-acid version can weigh close to 80 pounds. Same box, but the heft is very different.

Because the case size is standardized, a group 31 battery box or a group 31 battery tray built to BCI spec should fit any group 31 battery you pick. Still, check terminal placement and clearance under the lid before you order. Cables that are a half-inch too short have ruined more than one install.

Group 31 Battery Types Compared

Four chemistries dominate the group 31 category, and each one trades off price, weight, maintenance, and lifespan differently.

A group 31 AGM battery is where most buyers land. No water refills, decent tolerance for vibration and off-level mounting, and a lifespan that beats flooded batteries without much fuss.

An AGM group 31 battery also handles moderate to high discharge rates well, which explains why it turns up so often in marine and RV setups.

A group 31 lithium battery costs more upfront but delivers greater usable capacity per pound. Lithium tolerates deeper discharge without damage, so a 100-Ah lithium unit will often outperform a lead-acid battery with the same Ah rating.

Gel batteries round out the lineup. They handle extreme temperatures well and hold a charge over a long calendar life, though they typically deliver fewer deep-discharge cycles than AGM. They also don't like fast charging and need a charger profile built specifically for gel.

Group 31 vs. Other Battery Group Sizes

BCI group numbers describe a family of standardized case sizes. Within the same chemistry, physical capacity tends to rise as the group number climbs.

A group 24 vs. 31 battery comparison comes down to space and power. Group 24 is smaller, lighter, and cheaper, which works fine for light loads. Group 31 costs and weighs more, but it delivers meaningfully higher capacity and reserve capacity for sustained draws.

A group 27 vs. 31 battery decision is closer. Group 27 vs. 31 battery dimensions differ by less than an inch in length, but group 31 typically packs 10% to 20% more Ah. If your tray fits both, group 31 is the safer pick for anything power-hungry.

Starting, Deep Cycle, or Dual-Purpose?

Group 31 batteries are also built for different jobs, and matching the spec to your use matters as much as picking the chemistry.

• Starting batteries deliver a short, high-current burst to crank an engine, rated in cold cranking amps (CCA). A group 31 starting battery commonly ranges from 800 to 1,150 CCA.
• Deep cycle batteries deliver steady, lower current over long periods. A deep cycle group 31 battery is rated in amp-hours (Ah) and reserve capacity (RC), not CCA.
• Dual-purpose batteries split the difference. A group 31 dual purpose marine battery is common on boats that need engine starting and house power from one battery.

If you only need sustained power, a true deep cycle battery beats a dual-purpose unit. If you need starting and house power from a single battery, dual-purpose is the practical compromise.

Which Group 31 Battery Fits Your Application?

• Marine: A group 31 marine battery needs to handle vibration, humidity, and temperature swings. For house banks, a group 31 deep cycle marine battery or group 31 marine deep cycle battery in AGM or lithium is the standard pick. For combined duty, go dual-purpose.
• RV: A group 31 RV battery powers lights, water pumps, and appliances off-grid. AGM is the common baseline. Lithium is worth the premium if you camp off-grid often and want fast recharging from solar.
• Trolling motors: A group 31 trolling motor battery needs to sustain moderate current for hours without voltage sag. Lithium holds voltage steadier as it discharges, which keeps motor speed more consistent late in the day.
• Off-grid solar: A group 31 lithium marine battery, or its land-based equivalent, pairs well with solar charging. Lead-acid chemistries don't handle partial-state-of-charge cycling nearly as well, and they take longer to recharge.

A group 31 AGM deep cycle battery is still a solid, budget-friendly default across most of these uses when lithium's upfront cost doesn't fit the budget.

Key Specs to Check Before You Buy

Before adding a group 31 battery to your cart, run through these six checks.

• Physical fit. Measure your existing group 31 battery box or tray, including clearance for terminals and cables.
• Amp-hours (Ah). Higher Ah means longer runtime between charges. Most group 31 battery amp hours fall between 95 and 125.
• Reserve capacity (RC). Look for 150 to 230 minutes for solid backup runtime; some high-capacity AGM models run higher.
• Weight. Group 31 battery weight ranges from about 30 pounds for lithium to nearly 80 pounds for flooded or AGM. Confirm your compartment can handle it.
• Terminal type. A group 31 stud battery uses threaded posts; others use standard top posts or SAE terminals. Match your existing cables.
• Charger compatibility. AGM, gel, flooded, and lithium each need their own charging profile. The wrong one shortens battery life.

Match the Chemistry to the Job, Then Check the Fit

Group 31 only tells you the case size. What's actually inside comes down to chemistry and duty type. Flooded costs least but demands maintenance. AGM is the maintenance-free option most boats and RVs run on. Lithium costs the most upfront but earns it back in cycle life, weight savings, and charging speed.

Layer duty type on top of that. Starting batteries chase CCA, deep cycle batteries chase Ah and RC, and dual-purpose batteries split the difference for boats that need both from one unit.

Browse A1 SolarStore's current lineup of group 31 batteries, with specs listed up front so you can compare fit and performance before you buy.

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