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What’s the Right Size Battery for Your Trolling Motor? Simple, Clear Answers!

Written by: Pete D
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Battery selection for your trolling motor is critical, and the wrong choice can spell bad outcomes that range from underperformance to total failure.

Fortunately, once you understand the basics, it’s easy to figure out what you need.

We’ve had a lot to say about trolling motor batteries before, and if you want to know all the details, check out these articles:

We get questions like, “What’s the right size battery for my trolling motor?” all the time, and it’s easy to see why people get confused.

Not only are there lots of battery types out there - starter batteries, dual-use batteries, deep-cycle batteries, etc. - but there are different voltage outputs and numerous “group sizes” as well.

That’s a lot to wrap your head around, but a little battery know-how goes a long way.

If you want to know what size battery is right for your trolling motor, keep reading!

Also Read:

Battery Size: Voltage

All trolling motors are designed to be powered by either a 12V, 24V, or 36V system. And while there are single batteries that are rated for 24V or 36V, most anglers run either a single 12V battery or two or three in tandem, if necessary.

So right off the bat, you know you’ll need one, two, or three 12V batteries to run your trolling motor.

Typically, trolling motors that deliver 55 pounds of thrust or less need a single 12V battery. As you step up in power to motors that are rated to as much as 80 pounds of thrust, expect to power them with two 12V batteries. And for the most powerful trolling motors generating more than 80 pounds of thrust, you’ll typically need three 12V batteries to run them.

  • 55lbs of thrust or less = 12 volts (one battery)
  • 68-80lbs of thrust = 24 volts (two batteries)
  • 101-112lbs of thrust = 36 volts (three batteries)

The trolling motor you buy will be labeled as a 12V, 24V, or 36V motor, indicating how many 12V batteries it requires.

Keep in mind that adding more batteries won’t make a trolling motor more powerful. And using fewer batteries will, at the very best, result in dramatically less power.

There’s no way to cheat here!

Battery Size: “Group Sizes”

Here’s where things get tricky.

Let’s say that you now know that you'll need two 12V batteries to power your trolling motor. 

You go out and buy the best options for you.

When you come home, you look at your boat, see that there’s a battery compartment just waiting to be filled, and realize that the batteries you’ve bought won’t fit.

What gives?

The basic standards for battery sizing are created by an association called The Battery Council International (BCI). It’s just a trade board representing the interests of lead battery manufacturers, and “group sizes” are just a general guideline for how big batteries should be.

But to say that it’s a rough-and-ready standard is something of an understatement, especially once you consider the following:

  • it was designed around lead batteries, 
  • it specifies maximum rather than actual dimensions, and
  • battery manufacturers use these numbers as a very rough guideline.

Let’s look at common marine battery Group Sizes:

Group Size - Length - Width - Height

22NF9-7/16”5-1/2”8-15/16”

24M10-1/4”6-13/16”9-3/4”

259-1/16”6-7/8”8-7/8”

27M12-1/2”6-13/16”9-3/4”

31M13”6-13/16”9-7/16”

34M10-1/4”6-13/16”9-7/16”

359-1/16”6-7/8”8-7/8”

6512-1/6”7-9/16”7-9/16”

As you can see, as the Group Sizes get bigger, the batteries get bigger, too.

As crazy as this is, Group Sizes don’t match batteries’ dimensions in real life!

Unfortunately, maximum dimensions for a Group Size don’t tell you anything about actual dimensions, and battery manufacturers don’t usually obey these “guidelines.”

Worse still, in real life, batteries with a Group vary in size enough that they may not fit in a space labeled 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 OPT8016-103 Batteries D34M BlueTop Starting and Deep Cycle Marine Battery

Amazon 

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

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

Amazon 

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.

Final Thoughts

Finding the right “size” battery for your trolling motor takes a little know-how and some careful measurement. But once you understand the basics, you’ll get it right for the rest of your life.

We hope that you learned something from this article and that it answered the questions you had. 

If not, we’re always here to help!

Please leave a comment below, and we’ll be sure to answer you.

About The Author
Pete D
Born and raised in Cleveland, Ohio, Pete grew up fishing on the Great Lakes. When he’s not out on the water, you can find him reading his favorite books, and spending time with his family.
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