Batteries
Battery group size chart: what a BCI group number does and does not tell you
Quick answer
A BCI group number such as Group 31 defines the case dimensions and terminal layout, not the amp hour rating. This catalog carries a 36 volt 50 Ah lithium pack, a 120 Ah AGM, and a starting battery with no published amp hour rating, all in the same Group 31 case, which is why the group number alone never tells you the capacity.
BCI group sizes, U1, 24, 27, 31 and the rest, are a published case and terminal standard from Battery Council International. They tell an installer whether a replacement battery will physically fit the existing tray and reach the existing cables. They do not tell anyone the amp hour rating, the duty rating, or the cycle life, and treating a group number as a capacity specification is one of the most common mistakes made when shopping for a marine battery.
The table below pulls real batteries from this catalog, all in the same case sizes, to show exactly how much the capacity and duty rating vary within one group number. A Group 31 case alone shows up here as a 36 volt lithium pack, three different AGM capacities, and a pure starting battery with no amp hour figure published at all.
Group size examples across this catalog
Every row below shares a group number with at least one other row and nothing else in common.
Group 31 alone appears here as a 50 Ah 36 volt lithium pack, a 100 Ah AGM, a 120 Ah AGM, and a starting battery with no amp hour rating published at all, which is the proof that a BCI group number sets the case and terminal layout rather than the capacity.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Product | BCI group | Chemistry | Rated capacity or duty (published) |
|---|---|---|---|
| LiTime 12V 100Ah Group 24 LiFePO4 battery | 24 | LiFePO4 | 100 Ah |
| Renogy 12V 100Ah trolling motor LiFePO4 battery | 24 | LiFePO4 | 100 Ah |
| LiTime 12V 100Ah Group 27 LiFePO4 battery | 27 | LiFePO4 | 100 Ah |
| VMAX XTR27-110 12V 110Ah AGM Group 27 battery | 27 | AGM | 110 Ah |
| VMAX MR137-120 12V 120Ah AGM Group 31 battery | 31 | AGM | 120 Ah |
| Interstate Batteries Marine and RV Group 31 AGM, 12V 100Ah | 31 | AGM | 100 Ah, 925 CCA, dual purpose |
| Mighty Max MM-G31M 12V 110Ah deep cycle marine AGM | 31M | AGM | 110 Ah, 825 CCA, dual purpose |
| Odyssey ODX-AGM31 Extreme Series AGM battery | 31 | AGM | 1,150 CCA starting duty, no Ah published |
| LiTime 36V 50Ah TM LiFePO4 battery with low temperature protection | 31 | LiFePO4 | 50 Ah at 36 volts |
| VMAXTANKS V35-857 12V 35Ah AGM battery | U1 | AGM | 35 Ah |
Typical capacity ranges by group size
Where a typical range is useful for shopping, it is convention rather than a specification, and the note below says so plainly.
Group 31 typically runs 95 to 130 Ah in deep cycle service, but the table above already shows a real Group 31 lithium pack rated at just 50 Ah, so this range is a starting point for shopping, not a guarantee.
Rule of thumb Boating and fishing convention rather than a published standard. Taught everywhere, useful, and not a specification. Nothing enforces it and no body publishes it.
| BCI group | Typical deep cycle capacity range | Common marine role |
|---|---|---|
| U1 | 30 to 35 Ah | Small motors, portable sonar, canoes and kayaks |
| 24 | 70 to 85 Ah | Compact console boats, single battery kayaks with a larger box |
| 27 | 85 to 105 Ah | Mid size boats, a common trolling motor bank size |
| 31 | 95 to 130 Ah | The largest common case, full size bass and pontoon boats |
These are typical ranges repeated across the deep cycle market, not a specification. The group number governs the case dimensions and terminal layout. The amp hour rating is printed on the battery's own label and has to be read there, not assumed from the case size.
What a group number does and does not tell you
A BCI group number is a case and terminal standard, and this catalog carries a 36 volt 50 Ah lithium pack and a 120 Ah AGM in the same Group 31 case, which is the group standard doing its actual job.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| A BCI group number tells you | A BCI group number does not tell you |
|---|---|
| The physical case footprint, so it fits the existing tray | The amp hour rating, which the maker sets independently |
| The terminal position and type, so the existing cables reach | Whether the battery is built for deep cycling or for starting |
| That a same group replacement will physically fit | The reserve capacity, cold cranking amps, or cycle life |
Case size is not a duty rating
A starting battery built for a short, very high current burst to spin an engine and a deep cycle battery built to be drawn down slowly and recharged repeatedly use different internal plate construction, even when they share a case. A Group 31 automotive starting battery once surfaced in a marine parts search and was rejected from this catalog for exactly this reason: the case fit a marine battery tray, and the battery underneath it was still built to start an engine rather than run a trolling motor for a day. A dual purpose battery such as Mighty Max MM-G31M 12V 110Ah deep cycle marine AGM compromises both jobs at once rather than solving either one as well as a dedicated battery does.
Frequently asked questions
Does a Group 31 battery mean 100 amp hours?
No. Group 31 is a case size and terminal layout standard published by Battery Council International, not a capacity rating. This catalog alone carries a Group 31 case holding a 120 Ah AGM, a 100 Ah AGM, a 36 volt 50 Ah lithium pack, and a starting battery with no amp hour figure published at all. Always read the amp hour rating off the battery's own label rather than assuming it from the group number.
Can I swap a Group 24 lead acid battery for a Group 24 lithium battery?
The case will usually fit, since both share the same Group 24 footprint and terminal layout by definition. Capacity, weight and charging still need their own check: a lithium battery needs a charger with a lithium profile, and the amp hour rating on the new battery may differ from the one it replaces even though the group number matches, so confirm both before buying.
Is a Group 31 starting battery okay for a trolling motor?
No. A starting battery is built with thin plates for a short, very high current burst and does not tolerate the slow, deep, repeated discharge a trolling motor demands. The Group 31 case size only confirms it will physically fit the tray. The internal construction, which the group number says nothing about, decides whether the battery survives being run down and recharged all season.
What is BCI?
BCI is Battery Council International, the industry group whose group size standard defines the case dimensions and terminal position for common battery sizes such as U1, 24, 27 and 31. It exists so a replacement battery from a different brand still fits the same tray and reaches the same cables. It says nothing about amp hours, cold cranking amps or cycle life, which each maker sets on its own.
Why do two Group 31 batteries have different amp hour ratings?
Because the group number is a case standard, not a capacity standard. Two makers can build very different internal plate stacks, or an entirely different chemistry such as lithium, into the same outer case and still call it Group 31. The amp hour rating comes from the cells inside, not the sheet metal or plastic dimensions the group number actually governs.
What is Group U1 used for?
Group U1 is the smallest common marine and small equipment case, typically holding around 30 to 35 Ah in deep cycle service. It shows up on small transom motors, portable fish finders, ice fishing flashers and canoes, where a full size Group 31 would simply be too heavy and too large to carry, and the lower current draw of the equipment does not need the larger capacity anyway.
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Researched, not professional advice. This page is compiled from published engineering and regulatory figures, manufacturer specifications and owner-review consensus, not hands-on testing. Figures described as a rule of thumb are boating convention rather than published standards, and they are labelled that way wherever they appear. Marine electrical work is not house wiring. Use tinned, finely stranded marine cable, size it for voltage drop and for the ABYC E-11 ampacity table rather than one or the other, and put overcurrent protection within seven inches of the battery positive terminal, because everything between the post and the fuse is unprotected. A lithium battery needs a charger with a lithium profile. Never load a boat past its capacity plate, and remember that canoes and kayaks carry no federal capacity plate at all, so their stated capacity is the manufacturer own figure. Wear the life jacket, and treat early season water as the hazard it is: cold water immersion takes your breath and then your hands long before it takes your core.