What Size Marine Battery Do I Need for My Boat?

Your battery is one of the most vital parts of your boat’s electrical system. But all too many boaters don’t understand this essential item and find themselves asking, “What size marine battery do I really need?” There are a few simple ways to size up your marine battery needs. Join us as we take a closer look at everything you need to know about the batteries that keep your boat’s lights on and engine starting. 

What Is a Marine Battery? 

Marine batteries are a unique type of battery that, as their name suggests, are most often found on boats and other watercraft. In many cases, marine batteries can function as both boat starter and house batteries for limited energy use. This diverse set of uses is one of the hallmarks of this style of battery. There are different sizes of marine batteries available.

What Is the Difference Between a Marine Battery and Standard Battery?

While they may appear similar, there are some critical differences between a standard car starter and a deep cycle battery. Batteries chosen for marine uses should be rated to stand up to the abuses of time on the water. That means they should be fully sealed to prevent leaks, as well as be vibration resistant.

Lead-acid marine batteries are usually designed for deeper cycles than a standard car starting battery to allow them to perform starting and energy tasks. Depending on the size of the engine, this may require more batteries, however than a car. Marine batteries are also not rated at extremely cold temperatures but at the freezing point for maximum cold. This is because most boats don’t operate on icy waters.

How Do I Choose a Marine Battery?

There are a few important things to consider when shopping for a marine battery. First, think about what this battery will be powering. Is it solely for starting your boat and powering a few lights, or will there be significant amounts of electronics or appliances drawing from it as well? 

Smaller boats may be able to get away with one battery for both uses. But larger ones or ones with lots of power usage should opt for two separate batteries, one for starting the boat and another deep-cycle battery to power electronics and appliances.

In fact, we highly recommend using a second battery for power applications to minimize the chance of killing your start battery and stranding you on the water. In case you were wondering, all Battle Born Batteries meet these standards for deep cycle energy applications in marine environments.

Here’s how to choose the best marine battery for your boat.

What Size Marine Battery Do I Need for My Boat?

Sizing for a battery will depend on the application, either engine starting or deep cycle for energy use. Its highly recommended to have two battery systems on boats.

Engine Starting Requirements

Battery size requirements will naturally differ based on how you use your batteries. For starting batteries, you should consult the recommendations of the manufacturer of your engine. You should primarily look at the battery’s marine-cranking amps (also known as MCA). This measures how much power your battery can deliver in a short burst, measured at 32 degrees Fahrenheit. Ensuring your starting battery has sufficient MCA is vital for knowing you’ll have enough juice to start your engine in all conditions. 

A good rule of thumb for starting applications is to have 1 cranking amp per cubic inch for gasoline engines and 2 cranking amps per cubic inch for diesel engines. For example, a 6.0-liter crusader gasoline engine has about 366 cubic inches. A minimum of 400 marine cranking amps should be used to start this engine. If it were a diesel engine, 800 amps would be the minimum.

Auxillary or House Battery Requirements

Answering “what size marine battery do I need” is a bit more complicated when looking at auxiliary or house batteries. Power needs can vary widely based on the number and nature of things you’ve got plugged in. You’ll need to do some work on your own to figure out your watt-hour usage. 

Each device or appliance draws a certain amount of watts per hour while in use. Multiply this number by the hours (or minutes) you expect to use it between battery charges. Do this for every device and appliance, and add them to find your total watt-hours needed. It’s a good idea to get a battery that is larger than your baseline watt usage, just in case you go over.

Pro Tip: Read more on battery sizing in our article about Battery Sizes

Keep in mind that if using lead-acid batteries you would need to double the battery rating to match your energy needs as a discharge deeper than 50% damages the batteries rapidly.

Luckily, if you’re using high-quality lithium batteries as we manufacture here at Battle Born, deep discharges or complete drainage of the battery will not be a problem. Lead acid batteries of the past would get damaged every time they were deeply discharged, but our batteries do not suffer such problems. In fact, our batteries will last thousands of cycles more under all usage conditions than any lead acid battery competitor.

These days it’s highly recommended to use lithium batteries for energy storage needs because of the significant performance improvements they provide over lead-acid.

Can I Charge a Marine Battery With a Regular Charger? 

For traditional, old-school lead-acid batteries, a regular charger should do the trick. More modern lithium batteries like ours can also use standard chargers in many cases. However, opting for a dedicated lithium charger will speed up the process and fill the batteries more fully compared to a regular charger. While this may be an extra expense, many owners find it worthwhile to get the most out of their batteries.

We also recommend using a DC to DC charger or lithium alternator controller when charging lithium off an alternator. This is because lithium charges so much faster than lead-acid that the high draw can damage an alternator. A controller is needed to prevent damage and properly control the charge.

How Long Do Marine Batteries Last?

If you’re asking yourself “what size marine battery do I need,” you may also be wondering about how long it’ll be before you need to buy yet another. The length of time you can expect your marine battery to last will depend on several factors. These include the type of battery, how often you use it, and how well you keep up with maintenance. Traditional lead-acid batteries last as long as five or six years when cared for properly and used only moderately for engine starting. However, they can also fail in as little as a year or two if not cared for properly with regular maintenance. 

For lithium marine batteries, there’s no need to worry about maintenance. Simply ensure your battery isn’t damaged. Then, you can typically count on a decade or more of use from a quality lithium battery. While they may be somewhat more expensive than lead-acid batteries initially, lithium clearly makes up for this difference in an extended lifespan.

Get the Right Size Marine Battery for Your Boat

By now, you should hopefully have the answer to the question of what size marine battery you need. With this valuable information, you should have no trouble sizing up batteries and deciding between lead-acid and lithium. See you out on the water!