Marine Battery Sizes: Everything You Need to Know Before You Buy


The marine industry is experiencing impressive growth in the Asia Pacific. The number of newly registered boats in the Republic of Korea has doubled from previous years. Malaysia is steadily developing its coastline with berthing docks and marinas. 

Then there’s Australia with over 850,000 registered boats and over 40,000 storage spaces. The demand for marine batteries is expected to significantly rise in the coming years. 

Since all boats need a battery, there’s a wide variety of marine battery sizes available. Knowing the size you need is vital to ensure your new battery will fit on your boat and be strong enough to power everything you need.

Use this guide to help you buy your next marine battery. 

Marine Battery Sizes by Group 

Before you can start shopping for a battery, you need to know your boat’s electrical requirements. What’s the load that will be on the battery? 

There’re two parts to figuring this out. First, you need to figure out how much power you need to crank up the engine. Then you need to find out the amount of power it takes to run the boat’s electrical components. 

Battery Council International (BCI) 

The BCI created and maintains an international chart that defines the battery size groups. This is one of the specifications you need to look out for because not only does the battery need to produce enough power, but it also needs to fit in the space in your boat. 

There are many different boat styles on the water today. Each manufacturer creates its own space within the boat to store your battery or batteries. The battery you buy needs to fit in that space. 

Here is a list of the most common sizes: 

  • 22NF
  • 24M
  • 25
  • 27M
  • 31M
  • 34M
  • 35
  • 65

Each one of these categories has a defined set of dimensions that includes length, width, and height. That way, you know the exact size of the battery you are buying, regardless of the manufacturer you buy it from. 

The BCI tries hard to rarely make changes to the chart. However, they sometimes happen. If you are in the market for a new battery, it’s best to check the dimensions to ensure there haven’t been any changes. 

Find Your Size 

The first place you should look is on your old battery. There should be a spec label that tells you everything about the battery. You want to look for the number that has “BCI” next to it. 

This is your battery category size. You should buy a battery in this category to ensure that it fits in your boat. 

If you can’t find your category on the battery, then you should check your owner’s paperwork. The battery category will be listed here. If you don’t have the original paperwork, then your final option is to ask an expert or boat mechanic to help you identify your battery. 

Some boaters will tell you that it’s ok to move up or down the scale by a few categories. That particular section of the scale has one power strength; it’s just the dimensions that change. 

This can be risky as you need to make sure the dimensions fit the space on your boat. You want to make sure the battery will sit securely in the tray. You also need to check that your battery doesn’t interfere with anything around it. 

Amp Hours 

You will notice that the higher the category number, the more amp hours the battery has. Manufacturers do this on purpose. It means that the battery will have more power as the day goes on. 

The battery will stick with you and power your boat through the entire day. The last thing you want is to link back to the dock because your battery had inadequate amp hours. 

What About the “M”?

There are specific categories that have letters next to them. These designate a particular use for that battery. Those with an “M” are specifically designed for marine use. 

Now, this doesn’t mean you can’t use a battery from the other categories. But you need to understand that batteries not designed for marine use will not last as long. Their internal construction will not hold up to use on the water and being exposed to things like saltwater or salty air. 

You don’t want to leave yourself stranded because you bought a non-marine battery that rapidly corroded and failed. 

The Type of Battery You Buy 

It isn’t enough to buy a powerful battery in the right size. You need to purchase one that’s designed for the use you intend it for. There’re three types of batteries on the market that you will see, starting, deep cycle, and combo. 

You should always buy a replacement battery that’s the same type as the old one. 

Starting Power Cells 

These batteries can power all of your electronics and provide a burst of power to get your engine running. They charge slowly so they can produce a high current in short bursts. 

Deep Cycle Power Cells

These batteries do the exact opposite in that they provide a steady stream of power for a long time. They can offer longer runtimes and more power storage without needing a source of charge. 

Dual Purpose Power Cells

Most marine applications will call for the first two types of batteries. However, sometimes, there’s a need for a combo battery. These batteries act as a combination of the first two types. 

They work well in smaller boats that use one battery to both start the motor and run the electrical. They also work well in sailboats that require interchangeable batteries for their electrical loads. 

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Buy Your Marine Battery Today 

Now that you understand marine battery sizes, you’re ready to buy your boat a new battery. 

Start by looking at your old battery and determining the category and type. Next look for a new battery that matches your specifications. 

Browse our inventory of battery categories and buy your next marine battery today. 

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