What Size Battery Do I Need For My Boat?

Just like clothing, boat batteries aren’t “one size fits all”. Of course, “one size” items do exist, but we all know there’s no way they actually fit everyone!

That brings us to our next point– it’s  important to choose your battery size carefully. Why? Because not every boat is the same, and neither are every boater’s needs.The point of boat batteries is to optimize your boat’s performance, keep a trolling motor running smoothly, and power gadgets like your GPS or fish finder. Choose the right size battery for your boat, and you won’t damage your valuable equipment or run out of power.

Battery Size Basics

“What size battery do I need for my boat?” may be one of the first questions you ask when buying your boat or upgrading your existing batteries.

Boat dealers can likely help you with this. They may recommend certain batteries, and even install them for you. But what if you buy a used boat? Or what if you’re looking to add more on-board equipment, and your current batteries just won’t cut it?

Maybe you’ve decided to switch from lead-acid to lithium, and are wondering whether a smaller lithium battery can replace a large lead acid one. (Hint: the answer is yes!)

Whatever the case, here are the questions you’ll need to answer:

    • What kind of boat do you have? (For example, bass boats require bigger batteries than kayaks).
    • What types of batteries do you need? (Most bass boats need two types of 12 volt batteries: a starting battery, and a deep cycle battery).
  • What does the boat manufacturer recommend? 
  • How much amperage do you need?
  • Will you need more than one battery in parallel to run your equipment?

If you choose an underpowered or overpowered battery for your boat, you risk damaging your electronic equipment, and even the boat itself. So consider the questions above carefully.

What Size Battery Is Best For My Boat?

Ok, it’s time to get a bit more technical. Let’s dive deeper into some of the questions we mentioned above.

What Types Of Batteries Do You Need?

Your boat will need a cranking or starting battery to start its main engine. You’ll also need a deep cycle battery to power your trolling motor and electronic accessories. Add another deep cycle battery if you have dual engines.

There’s one more type of battery that can do both jobs: the dual purpose battery. But most experts agree this kind of battery doesn’t do either job well. You should only choose dual purpose if your boat doesn’t have enough space for more than one battery.

How Much Amperage Do You Need?

It’s time to crunch the numbers! You might not enjoy mathematical equations, but trust us–you’ll enjoy your fishing trips much more if you calculate the AH (amp hours) you need to operate your vessel.

Batteries have an Ah (amp hours) rating. This relates to the actual battery capacity. For example, a battery rated 100Ah delivers 5 amps for 20 hours, or 100 amps for one hour. But that may not always be true, since you shouldn’t discharge batteries past 50 percent of their capacity 

Carry out an “Energy Audit” and see what battery size will meet the energy demands of all your equipment. Here’s a simplified method:

  1. Use the formula W=AV. (Watts equals amperage multiplied by voltage.) Transpose this formula (divide the wattage by the system voltage) to get the number of amps for a certain device.
  2. Examples: If you use a 6 watt navigation light bulb in a 12 volt system, it will draw 0.5 amps. If you use it for 10 hours in a day, the light bulb will consume 5 Ah (amp hours). Most trolling motors use 5 to 30 amps an hour, depending on their load. So your battery setup needs to offer 30 to 200 amp hours.
  3. Finally, add up the average amps per hour your equipment will use to find out how many amp hours your batteries need to cover.

Common Questions

Here are our battery size recommendations for some of the most common bass boat setups:

What size battery do I need for my boat with a 24 volt trolling motor?

You can use two 12V 100Ah batteries for the trolling motor, and one 12V 150Ah battery for the engine starter and other onboard equipment.

What size battery do I need for my boat with a 36 volt trolling motor?

You’ll need one 36V 50Ah battery for the trolling motor, and one 12V 200Ah battery for your engine starter and other equipment. Or, you can swap the 36V 50Ah battery for three 12V 50Ah batteries in parallel.

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