If you have a motorized boat, you’ve probably heard of marine battery group size. But what does group size mean? And why should you care?
Have you ever been out on the lake, enjoying a day of fishing, and your battery dies? Having a battery with a bigger group size might have prevented the problem.
But here’s the thing:
Not all battery group sizes work for every boat.
And buying the wrong group size can cost you time and money.
So how do you know what group size is right for your boat?
How can you figure out the dimensions of a marine battery?
What are the different types of marine battery and which one do you need?
In today’s post, we’ll take a look at each of these questions.
We’ll also provide an easy reference chart with common marine battery group sizes and dimensions.
Are you ready to dive in?
Table of Contents
What is Group Size on Marine Battery?
The term “group size” refers to the dimensions and power capacity of a battery. It is used for all batteries, not just marine batteries.
Group size is usually shown as a mix of letters and numbers. For example, 24M. “24” is the size and “M” stands for “marine.”
Why does battery group size matter?
For one thing, it helps you find the right designation. Installing a battery not rated for marine use will kill your battery quicker than anything else.
But even a marine battery may die on the water if you aren’t using the best group size for your boat.
In general, a larger group size means the battery will last longer. It will also be able to simultaneously power more functions on your boat such as fish finders, radios, and pumps.
Many boats are able to support more than one group size. If you want to install a group size that’s larger than your battery tray, you may be able to replace the battery tray.
Some common marine battery group sizes include:
- 24M
- 25
- 27M
- 31M
- 34M
- 35
What Are the Dimensions of a Marine Battery?
The dimensions of a marine battery refer to its length, height, and width.
You might be wondering how to find the dimensions of your current marine battery. Is it fairly small or the largest your boat can handle?
The battery dimensions are probably listed on the battery. If you can’t find them, use a measuring tape to measure each side of the battery. This will give you the dimensions.
Here are a few examples of marine battery dimensions:
- 24M: 10 1/4 ” L, 6 13/16” W, 9 3/4” H
- 27M: 12 1/2” L, 6 13/16” W, 9 3/4” H
- 31M: 13” L, 6 13/16” W, 9 7/16” H
How Do I Know Battery Group Size?
So now you understand battery group size and dimensions. How do you determine your battery’s group size?
As with the dimensions, the group size should be listed somewhere on the battery label. Compare the numbers and letters against those listed above or in the chart at the end of this article.
If you can’t find the group size on the battery, refer to your boat’s owner’s manual. The manual should provide all the specs you need about the boat, including acceptable battery group sizes.
Remember, your boat may support more than one group size. This is because the actual dimensions of each group size are often quite similar.
You might have noticed that in the dimensions listed above.
Though the sizes may be similar, the power capacity is generally better in larger group sizes. The higher the number, the longer your battery will last.
Is There a Difference Between Deep Cycle and Marine Battery?
So what does deep cycle mean?
You’ve heard the term thrown around when talking about marine batteries. You’ve probably also heard terms like “starting,” “cranking,” and “dual purpose.”
Why should you care?
All of these terms refer to different types of batteries that your boat may use.
So no, there isn’t exactly a difference between the terms “deep cycle” and “marine battery.” A deep cycle battery is a type of battery, and a deep cycle marine battery is a specific type of marine battery.
Let’s take a closer look at deep cycle, starting or cranking, and dual purpose batteries as they apply to marine batteries.
Deep Cycle Batteries
Deep cycle batteries can be “deep cycled,” or discharged, repeatedly without the battery becoming damaged. They provide power for long periods.
Some common uses for deep cycle marine batteries include:
- Navigation aids
- Trolling motors
- Lights and radios
Starting or Cranking Batteries
Starting batteries are sometimes called cranking batteries. They generally provide a short burst of power used for starting your engine, then provide lower amperage reserves for other functions.
Starting batteries are also used for:
- Fish finders
- Pumps
- Other accessories
Dual Purpose Batteries
Dual purpose batteries are a cross between deep cycle and starting batteries. They can be used both for starting the engine and for performing other functions while the boat is running.
Most boat batteries are dual-purpose because they are the most versatile.
What Size Battery Do I Need For My Boat?
So now we come to the big question: which size battery is best for your boat?
The answer may depend on a number of factors.
What battery sizes and types does your boat use? This is the most important question to figure out. Again, battery information can be found in your boat’s owner’s manual and on battery labels.
How often do you use your boat? If you go boating often, you’ll want a tough, long-lasting battery that can handle all your power needs.
How long do you stay out on the water? Staying out for most of the day may drain smaller group size batteries.
Has your current battery been dying a lot lately? If you frequently find yourself stranded out on the water, it may be time for a new battery.
If your boat supports a larger battery than the one you have now, it wouldn’t be a bad idea to upgrade to that larger battery.
Here’s a chart of common marine battery group sizes and dimensions according to Dennis Allen’s Affordable Marine Service:
| 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” |
Conclusion
Choosing the right marine battery for your boat means more than just knowing the group size.
You should also consider the amount of time you spend on the water, the boat functions powered by your battery, and whether or not your boat can support a larger battery than the one you have now.
Using the chart in this guide, you can easily determine the battery group size you have as well as others you might be able to use.
Just remember to always consult your owner’s manual before using a battery in a different group size.
And there you have it! The next time you head to the lake, you can go out with confidence knowing you’ve installed the best marine battery group size for your boat!
Read more: How Heavy Is A Trolling Motor + Battery?
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