What the M in Group 24M, 27M and 31M Means

September 8, 2026 · Marine Battery Source

Close-up of marine battery posts with tapered lead terminals and threaded studs with wing nuts

If you’ve shopped for a boat battery, you’ve probably seen labels like 24M, 27M and 31M, or Interstate’s 4DM. They look like group sizes with an extra letter, and that’s essentially what they are. This guide explains what the “M” means, how marine batteries differ from automotive batteries of the same size, and what to check when comparing M-labeled batteries.

The short answer

The M stands for marine. A 27M battery is a marine version of a group 27 battery. The case dimensions are in the same family, so a 27M generally fits where a group 27 fits. The difference is in the features: terminals, construction and ratings meant for boats.

What marine versions usually add

Dual terminals. Many M batteries have both standard round posts and threaded studs. The posts accept automotive-style clamps, while the studs take ring terminals secured with wing nuts or hex nuts. Ring terminals on studs hold better under vibration and are easier to stack for multiple circuits.

Vibration resistance. Boats pound in waves, so marine batteries are built to handle vibration and shock better than many car batteries.

Ratings for marine use. Marine batteries usually list MCA (marine cranking amps, measured at 32°F) along with CCA (measured at 0°F), and reserve capacity, which matters for running electronics and pumps.

Deep-cycle and dual-purpose designs. Car batteries are almost always starting batteries. Marine M batteries come in starting, dual-purpose and deep-cycle versions.

At-a-glance summary: The "M" in 24M, 27M or 31M marks a marine version of that BCI group size, with a similar case size; Marine versions typically add dual terminals, threaded studs plus posts, and are built for vibration and deep cycling; The M tells you about fit and terminals, not capacity; always compare CCA, MCA, reserve capacity and amp-hours — Key takeaways: What the M in Group 24M, 27M and 31M Means

Examples from our store

Model	Group	Type	Capacity	CCA	Reserve
Interstate 24M-XHD	24M	Starting	Not listed	800	135 min
Interstate SRM-24	24M	Deep cycle	81Ah	550	140 min
Interstate 24M-AGM-A	24M	AGM dual purpose	70Ah	750	140 min
Interstate 27M-AGM	27M	AGM dual purpose	85Ah	850	182 min
Interstate SRM-27	27M	Deep cycle	88Ah	600	160 min
Interstate 31M-AGM	31M	AGM dual purpose	100Ah	925	200 min
Interstate SRM-31	31M	Deep cycle	98Ah	675	210 min
Interstate 34M-AGM	34M	AGM dual purpose	62Ah	800	125 min
Interstate SRM-4D	4DM	Deep cycle	195Ah	1,314	390 min

Interstate also uses M sizes for lithium: the Li24M, Li27M and Li31M models fit marine 24, 27 and 31 spaces.

The M doesn’t tell you capacity

Two 27M batteries can be very different. In the table above, the 27M-AGM lists 850 CCA and 85Ah, while the SRM-27 lists 600 CCA and 88Ah. One is built for starting plus accessories; the other for deep cycling. Always look past the size label to the ratings:

CCA and MCA for starting power.
Reserve capacity (minutes at 25A) for running loads with the engine off.
Amp-hours for deep-cycle capacity.
Chemistry: flooded, EFB, AGM or lithium.
Warranty.

M vs automotive batteries

Can you put a car battery in your boat? In an emergency, maybe, but it’s not a good idea. Car batteries lack dual terminals, aren’t built for marine vibration, often have no reserve or deep-cycle rating, and can wear out quickly if discharged by electronics or a trolling motor. Marine batteries also come with marine-specific warranties. A boat inspection or survey may flag an automotive battery.

M vs non-M marine batteries

Some marine batteries, like Trojan’s 24TMX and 27TMX, use the plain group number. These are still marine deep-cycle batteries; Trojan just doesn’t use the M suffix. Compare dimensions on the listing to confirm the fit.

Terminal tips for M batteries

Use ring terminals sized for the stud, commonly 3/8 or 5/16 inch.
Put the heaviest cable, usually the main positive or starter cable, closest to the battery case, with smaller ring terminals on top.
Don’t use wing nuts on high-current connections if your installer recommends hex nuts with lock washers.
Cover positive terminals with boots to prevent accidental shorts.
Coat terminals with a protectant spray on saltwater boats.

Do lithium batteries use M sizes?

Some do. Interstate’s lithium models use 24M, 27M and 31M labels. Other lithium makers list plain group sizes like “group 24” or “group 31,” or describe a “group 27/31” fit. Lithium cases can differ slightly in height or terminal layout from lead-acid, so check the dimensions on each listing.

Fitting a different size

If you want more capacity, you might be able to move from a 24M to a 27M or 31M, but only if the tray, box and hold-down allow it. See how to measure a battery compartment before ordering.

Why the M matters at the counter

When you buy a replacement, telling the store “27M” narrows things down to marine batteries that fit a group 27 space. That saves time. But it’s only a starting point: ask about the battery’s job, whether it’s starting, dual purpose or deep cycle, and the ratings you need. If you’re unsure, bring photos of your old battery’s label and the terminals.

Reading the label

A typical marine battery label shows the group size (such as 27M), CCA, MCA, reserve capacity and sometimes amp-hours, along with a date code. Check the date code when buying: a battery that has sat on a shelf for a long time may need a full charge before use. Lead-acid batteries self-discharge over time, so fresh stock is best.

Products mentioned

Frequently asked questions

What does the M mean in 27M?

It marks a marine version of the group 27 size, usually with dual terminals and marine ratings.

Is a 27M the same size as a group 27?

It’s in the same size family and generally fits a group 27 space. Check the listing for exact dimensions.

Can I replace a 24M with a 27M?

Only if your tray and hold-down fit the longer battery. Measure first.

Does 31M mean more cranking amps?

Not by itself. The M describes the size and features, not the ratings. Compare CCA and MCA.

Can I use a car battery on my boat?

It’s not recommended. Car batteries aren’t built for marine vibration or deep cycling.