Group 24 vs 27 vs 31 Marine Batteries
Group 24, 27 and 31 are the three most common marine battery sizes. If you’re replacing a battery or upgrading, you’ll probably choose among them. The group number tells you the case size, not the battery’s capacity or quality, but each size tends to come with a typical range of ratings. This guide compares the three sizes and helps you choose. Always check the exact dimensions on the product page and measure your space.
Approximate sizes
| Group 24 | Group 27 | Group 31 | |
|---|---|---|---|
| Length (approx.) | 10.3 in | 12 in | 13 in |
| Width (approx.) | 6.8 in | 6.8 in | 6.8 in |
| Height (approx.) | 8.9 in | 8.9 in | 9.4 in |
These are typical BCI dimensions; actual batteries vary slightly, and terminals and handles add height. The main difference is length: a group 27 is about 1¾ inches longer than a group 24, and a group 31 is about another inch longer and a bit taller.
Typical lead-acid ratings by size
Using in-stock lead-acid models from our listings:
| Group 24 | Group 27 | Group 31 | |
|---|---|---|---|
| Deep-cycle capacity | 76–100Ah | 85–115Ah | 98–104Ah |
| Reserve capacity @ 25A | 137–150 min | 158–200 min | 177–210 min |
| Cranking (CCA, where listed) | 400–800 | 600–850 | 675–925 |
| Weight (where listed) | 47–54 lb | 55–66 lb | 67–69 lb |
Ranges overlap because chemistry and design matter as much as size. Trojan’s flooded SCS200 in a group 27 case lists 115Ah, more than some group 31 AGM batteries.
Lithium changes the picture
LiFePO4 batteries pack much more usable energy into each size and weigh far less:
| Group 24 lithium | Group 27/31 lithium | Group 31 lithium | |
|---|---|---|---|
| Examples | EPOCH 12V 105Ah (22.1 lb), Dakota DL+ 135Ah (27.2 lb) | Battle Born 100Ah (31 lb), Interstate Li27M 100Ah | EPOCH 12V 300Ah (58 lb), EPOCH 12V 172Ah dual purpose (40 lb) |
A group 31 lithium battery can hold 300Ah or more, roughly six times the usable energy of a group 31 lead-acid battery. With lithium, you choose the group size for fit and the capacity for your needs.
Which size for which job?
Group 24 suits small boats, starting batteries for small and mid-size outboards, and electronics batteries. It’s the lightest and most compact, which matters on jon boats, skiffs and kayaks. Many 12V lithium trolling batteries use a group 24 case.
Group 27 is the most popular deep-cycle size for trolling motors and dual-purpose use on bass boats, pontoons and center consoles. It offers more reserve than a 24 without a big jump in length.
Group 31 suits larger dual-purpose and house batteries, high-cranking starting for bigger outboards, and many native 24V and 36V lithium trolling batteries. It often uses threaded stud terminals.
Can I go up a size?
Yes, if it fits. Moving from a group 24 to a 27 or 31 gives more capacity, but you need:
- Length: the box or tray must be long enough.
- Hold-down: the strap or bracket must fit the new size.
- Cable reach: cables must reach the terminals, which may move.
- Terminal type: group 31 batteries often use studs instead of posts, which may need new cable lugs.
Never install a battery that doesn’t fit the tray securely. A loose battery is a hazard in rough water.
Can I go down a size?
Also yes, with a spacer or a smaller tray, as long as the battery is held firmly. This is common when switching to lithium: a group 24 lithium battery may have more usable energy than the group 27 lead-acid battery it replaces, at half the weight.
Choosing by use
| Use | Lead-acid pick | Lithium pick |
|---|---|---|
| Small outboard starting | Group 24 starting or dual purpose | Dakota DL+ 60Ah (group 24) |
| 12V trolling, small boat | Group 27 deep cycle | Group 24 100–105Ah lithium |
| 24V/36V trolling bank | Two or three group 27 or 31 | Single 24V or 36V lithium (often group 27 or 31 size) |
| Dual purpose, single battery | Group 27 or 31 AGM | Dakota DL+ 135Ah or EPOCH 172Ah |
| House battery | Group 31 deep cycle | Group 31 300Ah lithium |
Weight and balance
Weight climbs with size in lead-acid: from under 50 pounds for a group 24 to nearly 70 for a group 31. On a small boat, especially with two or three batteries for a trolling bank, that adds up fast and affects how the boat sits and planes. Place heavy batteries low and close to the centerline. If weight is a concern, lithium in any of these sizes typically saves 20 to 40 pounds per battery.
Marine “M” sizes
You’ll often see 24M, 27M and 31M. The M indicates a marine version, typically with dual terminals: threaded studs plus standard posts. The case size is the same family. See what 24M, 27M and 31M mean for details.
How to measure
Before ordering, measure the inside of your box or tray: length, width and usable height, including clearance for cables and the lid. See how to measure a battery compartment for a step-by-step guide.
More on each size
Read the full guides to group 24 marine batteries and group 27 marine batteries. For an overview of all BCI sizes, see BCI battery group sizes explained. For lithium-specific sizing, read lithium battery group sizes. If you’re unsure which type of battery you need, start with cranking vs dual-purpose vs deep-cycle and the marine battery buying guide.
Products mentioned
- Trojan 24TMX (group 24)
- Trojan SCS200 (group 27)
- Interstate 31M-AGM (group 31)
- EPOCH 12V 105Ah (group 24 lithium)
- EPOCH 12V 300Ah (group 31 lithium)
Frequently asked questions
Is a group 27 battery bigger than a group 24?
Yes, mainly longer, by about 1¾ inches, with room for more capacity.
Will a group 31 fit in a group 27 box?
Usually not. A group 31 is about an inch longer and a bit taller. Measure first.
Which group size has the most amp-hours?
In lead-acid, group 31 and large group 27 deep-cycle batteries are similar, around 100–115Ah. In lithium, group 31 cases can hold 300Ah or more.
Can I replace a group 27 with a group 24 lithium?
Often yes, if you secure it properly. Many group 24 lithium batteries offer more usable energy than a group 27 lead-acid battery.
Does group size affect cranking amps?
Indirectly. Larger cases fit more plate area, so CCA tends to rise with size, but design matters too.