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Marine Battery Group Size

Selecting a marine battery is typically an expensive proposition, and it’s critical to get the details right.

That’s why we’ve spent so much time discussing this topic. One thing you’ll notice is that there are lots of technical specifications to keep in mind: battery types, Amp hours, cold cranking amps, reserve capacity, weights … the list gets pretty long, pretty fast.

And one thing we note every time we discuss battery selection is size, specifically, Group Size in relation to real-world measurements of height, length, and width.

That may leave you asking questions like, “What is Group Size?” and “Why can’t I rely on it without measuring my battery?”

We’ve got answers - so keep reading!


Group Size Explained

The Battery Council International (BCI) is a trade board designed to promote the interests of lead battery manufacturers, and as part of their work over the more than 100 years they’ve been around, they’ve offered a basic standard for battery sizing. 

And basic it is.

Group Size is simply a set of sizing categories with maximum dimensions for each group.

It doesn’t specify anything beyond that, which can create significant problems in the real world, problems further complicated by the manufacturer’s reluctance to actually follow these standards.

Let’s look at common marine battery Group Sizes:

Group Size Length Width Height
22NF 9-7/16” 5-1/2” 8-15/16”
24M 10-1/4” 6-13/16” 9-3/4”
25  9-1/16”  6-7/8”  8-7/8”
27M 12-1/2” 6-13/16” 9-3/4”
31M 13” 6-13/16” 9-7/16”
34M 10-1/4” 6-13/16” 9-7/16”
35 9-1/16” 6-7/8” 8-7/8”
65 12-1/6” 7-9/16” 7-9/16”

As you can see, as the Group Sizes get bigger, so too do batteries, and typically, this also means an increase in capacity.

But Group Sizes Don’t Match Batteries in Real Life!

So far, so good.

But a problem comes in quickly when you realize three things about this sizing system:

  1. It was designed around lead batteries, 
  2. It specifies maximum rather than actual dimensions, and
  3. Battery manufacturers use these numbers as a very rough guideline.

Let’s break these down and see where the issues lie.

Traditional wet-cell lead/acid batteries need to be pretty big to supply much power. That’s because the tech depends on thick lead plates bathed in an acid catalyst to deliver power. The more massive the plates, the more power they can store.

But more modern battery tech, such as lithium-ion batteries, simply don’t require as much space - or mass - as these older systems. Hypothetically, lithium-ion marine batteries could be made substantially smaller than their lead-based counterparts, but to make Group Sizing more or less consistent, battery manufacturers increase the dimensions of their lithium batteries to make them fit the lead/acid Group Sizes with roughly equivalent power specifications.

That makes replacing a lead/acid battery with a lithium-ion battery easier, as you’ll typically be able to fit your new high-tech battery in the same space as the old one.

But not so fast!

The problem here is that maximum dimensions don’t tell you anything about actual dimensions, especially since battery manufacturers don’t really obey these guidelines.

Keep in mind that “fit” isn’t just a question of there being a gap between the battery and the box or compartment, but also where that space is and how big or small it is. And in real life, batteries with a Group vary enough that they may not fit in a space designed for them at all.

For example, let’s look at two Group 34M batteries.

The Optima Blue Top measures L 10.6” x W 6.88” x H 7.94.”

OPTIMA Batteries OPT8016-103 D34M BlueTop Starting and Deep Cycle Marine Battery


The Banshee measures L 10.2” x W 6.9” x H 7.9.”

Marine Battery Group 34 Replaces 34M, 8016-103, SC34DM


Right off the bat, you’ll notice that the Optima is almost ½” longer than the Banshee but very close in width and height. That ½” could pose a problem, but it’s not that serious, right?

Except that the Group 34M supposedly maxes out at L 10.25” x W 6.81” x H 9.43!”

The Optima is easily within the height maximum, well over the length maximum, and just over the width maximum.

And the Banshee?

No better; while it’s within the height maximum and just within the length maximum, it’s just over the width maximum.

So if you bought a battery box designed for a Group 34M battery, neither of these batteries might fit, and ditto for a battery compartment sized to fit the Group 34M maximums.

That’s why you should measure every battery, every time - and never rely on Group Size to ensure a good fit.

What Does This Mean for You?

Always measure.

It’s as simple as that.

You simply can’t depend on a battery’s Group Size to tell you anything more than a rough approximation of its dimensions, and you certainly can’t rely on it for a precise fit.

Yeah, that’s crazy. We agree, but there it is.

We hope that this article has helped you figure out what Group Sizes mean - and what they don’t mean - and if you’ve learned something, we’d love to hear from you.

Please leave a comment below!

About The Author
John Baltes
If it has fins, John has probably tried to catch it from a kayak. A native of Louisiana, he now lives in Sarajevo, where he's adjusting to life in the mountains. From the rivers of Bosnia to the coast of Croatia, you can find him fishing when he's not camping, hiking, or hunting.