Marine Battery Group Sizes

If you run a trolling motor on your boat or any other accessories that need to be powered (stereo, fish finder, etc), you will need a marine battery. While searching for batteries, you will come across the term battery group size. But what are battery group sizes exactly?

What is the meaning of Battery Group Size?

The term “battery group size” simply refers to the battery’s physical case size. It is a specification that you’ll find on all boat batteries.

It is basically the physical measurements of the battery casing as set by the Battery Council International (BCI). The height, width, and length of a battery specify its group size.

You should be able to locate the group size of your battery on the labels.

Here are a few things to keep in mind about battery group sizes:

  • A smaller group size generally indicates a larger battery.
  • Conversely, a larger group size generally indicates a smaller battery.

Table of common battery group sizes used in boats

Here is a table of the most common battery group sizes that you will use on inflatable boats and traditional boats.

Battery group sizeL (inch)W (inch)H (inch)L (mm)W (mm)H (mm)
U17 3/45 3/167 1/8197132181
21 (R)8 3/166 13/168 3/4208173222
22F9 1/26 7/88 5/16241175211
22HF9 1/26 7/89241175229
22NF9 7/165 1/28 15/16240140227
22R96 7/88 5/16229175211
2410 1/46 13/168 7/8260173225
24F10 3/46 13/169273173229
24H10 1/46 13/169 3/8260173238
24R10 1/46 13/169260173229
24M(T)10 1/46 13/169 3/4260173248
259 1/166 7/88 7/8230175225
26 (R)8 3/166 13/167 3/4208173197
2712 1/166 13/168 7/8306173225
27F12 1/26 13/168 15/16318173227
27H11 3/46 13/169 1/4298173235
27M12 1/26 13/169 3/4318173248
29NF135 1/28 15/16330140227
31M136 13/169 7/16330173240
3313 5/166 13/169 3/8338173238
34 (R)10 1/46 13/167 7/8260173200
34M10 1/46 13/169 7/16260173240
359 1/166 7/88 7/8230175225
36R10 3/87 1/48 1/8263183206
40R10 15/166 7/86 7/8277175175
4111 3/166 7/86 7/8293175175
429 5/166 13/166 13/16243173173
4313 1/86 7/88 1/16334175205
459 7/165 1/28 15/16240140227
4610 3/46 13/169273173229
479 11/166 7/87 1/2246175190
4812 1/166 7/87 9/16306175192
49156 7/87 3/16381175192
5013 1/2510343127254
519 3/85 1/168 13/16238129223
51R9 3/85 1/168 13/16238129223
527 5/165 13/168 1/4186147210
53134 11/168 1/4330119210
547 5/166 1/168 3/8186154212
558 5/86 1/168 3/8218154212
56106 1/168 3/8254154212
578 1/167 3/166 15/16205183177
5810 1/167 3/166 15/16255183177
58R10 1/167 3/166 15/16255183177
5910 1/167 5/87 3/4255193196
6013 1/166 5/168 7/8332160225
617 9/166 3/88 7/8192162225
628 7/86 3/88 7/8225162225
6310 3/166 3/88 7/8258162225
6411 11/166 3/88 7/8296162225
6512 1/167 1/27 9/16306190192
708 3/167 1/167 11/16208179196
718 3/167 1/168 1/2208179216
729 1/167 1/168 1/4230179210
739 1/167 1/168 1/2230179216
7410 1/47 1/48 3/4260184222
759 1/167 1/167 11/16230179196
7613 1/87 1/168 1/2334179216
7810 1/47 1/167 11/16260179196
859 1/166 13/168230173203
869 1/166 13/168230173203
909 11/166 7/86 7/8246175175
91116 7/86 7/8280175175
9212 1/26 7/86 7/8317175175
93156 7/86 7/8354175175
95R15 9/166 7/87 1/2394175190
96R9 9/166 13/166 7/8242173175
97R9 15/166 7/87 1/2252175190
98R11 3/166 7/87 1/2283175190

Group size considerations when purchasing new batteries

When it comes to deep cycle marine batteries, a larger group size implies that you should have a higher Amp Hour (Ah) capacity.

If you realize that your trolling motor is weak by the time you are finishing up on a day of fishing in windy conditions, you’d be wise to upgrade and get a larger Ah battery. This will mean that the new battery will have a different group size.

Many battery group sizes are very similar, differing by fractions of an inch. These batteries can be used interchangeably, if:

  • the new battery fits the ‘old’ battery’s tray,
  • the new battery has equal or better charging and discharging characteristics than to the old battery.

The following are notable considerations to make when shopping for a new battery:

  • Find marine batteries that have Group Size with the letter “M” in the name. This is what the BCI designate for use in marine boats and RVs. Marine batteries have a distinct construction that differs from automotive batteries.
  • Consider the available space in your boat prior to buying the battery. There is not much free space in inflatable boats, especially if you have lots of gear. With traditional boats, the battery has to be compatible with the battery tray size. Alternatively, you can install a bigger tray for larger group size.
  • If your issues with powering the electronics and accessories over a whole day persist, you may have a wiring fault. Confirm if your boat has enough wire gauge to support the draw of current from your electronics.
  • You might also want to add a dedicated dual-purpose battery which will exclusively power the fish finders on your boat and leave the less demanding components to the normal cranking battery.
  • On traditional boats, be sure to install a proper onboard marine battery charger so that your battery investments are well cared for and maintained.

Basic Types of Boat Batteries

Generally, there are two kinds of 12-volt batteries.

  1. The first one is the cranking/starting battery, which is tailored to start the main engine.
  2. The other one is the deep-cycle battery, which is used for powering electrical accessories like fish-finders, trolling motors, and radios.

You can also find dual-purpose batteries that perform both of these functions.

Batteries can further be categorized by their construction and conducting medium:

  • Wet cell battery (aka flooded battery)
  • Gel battery
  • Absorbent glass mat (AGM) battery

These all have pros and cons that you should take into account before buying.

It is highly advisable to use only AGM batteries on boats, because they are spillproof, maintenance free, and shock resistant.

If you want to learn more about marine batteries, have a look at my guide on trolling motor batteries.

Are you new to the world of inflatable boats? Then my Getting Started Guide is for you. You’ll find tips, tricks, and how-to articles to start off right.

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