Photo courtesy of Nosk Lithium.

Out of sight, out of mind, that’s the normal stasis when it comes to marine batteries. Turn the key, hear your engine awaken, and point the bow toward your destination. As long as there’s juice enough to get where you’re headed and back there’s not much to worry about. Tucked away in their generally dank and inconspicuous compartments, marine batteries simply go about performing their task of providing electrical power to your engine, dashboard, electronics package, accessories and more.

Until they don’t – and that’s the problem. Keeping your batteries well-charged, ensuring their connections remain corrosion free, and checking power levels before even cranking the engine saves a lot of headaches in the long run. All batteries eventually degrade making replacement necessary. Understanding how to pick a boat battery that will adequately fit your needs becomes the driving question. And its answer requires a little more thought than you might expect as there are several choices available these days. Here’s a simple break-down to help you begin unraveling the mystery of buying a marine boat battery.

Where Do You Start?

When it comes to buying a new marine boat battery the best way to get started is to crack open your boat owner’s manual. Here you can check your vessel’s recommended battery size, ratings and type. If your vessel has run well using the suggested batteries, simply replacing your current one with the same model may be a good idea.

There are, however, some conditions that may make it worthwhile to consider other options. Perhaps you’ve upgraded your electronics package and it now draws considerably more amperage. Maybe you’d like a battery that weighs less and is easier to remove from your vessel when necessary. Perhaps you need a unit that is more durable in a high-vibration environment. Or maybe you want a battery with a longer life expectancy. In each of these cases, a chat with a local marine mechanic is probably a good idea. Eventually, though, it’s going to be your call so the more you know the better you’ll feel when it comes time to lay your money down.

Types Of Marine Batteries

The first thing you’ll need to determine is whether you require a marine starting, deep cycle or dual-purpose battery.

Starting Batteries: Also known as boat cranking batteries, have a single purpose and that’s to fire-up the engine. They discharge high amperage current bursts for several seconds when you activate the ignition sequence and are then quickly recharged by the engine’s alternator. They are not used to run electronics, lights, electronics packages, etc.

Deep Cycle Batteries: These batteries keep the boat running throughout your trip, providing power to the lights, GPS, fish-finders, navigation tools, trolling motor and accessories. A deep cycle battery is designed to fully recover after longer discharges that would, in short order, draw down and eventually ruin a starting battery.

Dual Purpose Batteries: Dual-purpose marine batteries combine engine starting ability with the function of a deep cycle battery. While saving space, weight and money this single battery can do the task of two. This battery type is a good fit for many smaller vessel styles but be aware it drains quicker than deep cell batteries. Which may not have the capacity to produce enough juice to crank some powerhouses exceeding 200 hp.

Battery Chemistry

All of the above marine battery types are available with different internal chemistries including Flooded Lead Acid (FLA), Gel, Absorbed Glass Matt (AGM) and, Lithium Iron Phosphate batteries (LiFePO4).

Flooded lead acid batteries (FLA):

Internally, their main components are lead plates and an acid mixture of distilled water and acid. They are the least expensive of marine batteries and do a good job as long as you never drain them below 50-percent capacity. They require a little extra work, but given proper maintenance, they reliably get the job done.

However, these batteries are bulky, have a life expectancy of just two to three years, require off-season recharging and occasional topping-off of the fuel cells with distilled water. They are heavy (an average 100-Ah lead acid battery weighs between 60 and 80 pounds), more fragile than competing formats, and may not hold up well in a high-vibration environment. Lastly, they charge more slowly compared to other battery options.

Marine gel batteries and AGMs:

These are actually types of FLA batteries. Somewhat more expensive, they are fully sealed so there’s no need to top them off. They don’t leak, and they have a slower discharge rate than the original FLA format. Still, there’s that heavy weight and bulkiness issue, plus limited life span, to consider.

Lithium iron marine batteries (LiFePO4):

Although these are newer to the market, they address most FLA shortcomings. Exceptionally durable and fully sealed, they weigh half as much as flooded lead acid batteries and last 10 to 12 years. While FLA batteries risk damage if drawn down below 50-percent of full charge, a quality lithium marine battery will allow 100% DoD (Depth of Discharge). Significantly increasing run times, with no ill effects. Also, since they provide nearly double the capacity, you may only need half the number of batteries, which further reduces weight.

“They are a better mouse trap,” explains, James Holst, chief marketing officer at Norsk Lithium, a leading manufacturer of lithium batteries. “This battery type eliminates all the headaches of FLA models. Plus most manufacturers provide apps that allow owners to check cell voltage and discharge rates so you always know exactly how your batteries are performing.”

FLA vs. LeFePO4

The big drawback here is that although lithium batteries can last four times as long as FLA models, they cost three times as much. You’ll certainly see significant cost savings over time but you’ll need to cough-up more money up front. Then again, you may only need half as many batteries to run your craft compared to using FLAs, which could reduce your sticker shock. Lithium batteries are drop-in replacements in many instances, meaning they’re designed to fit like an FLA in your boat’s battery compartment. Also note that, in some areas lithium batteries may be hard to recycle.

Marine Battery Sizes

“What size battery do I need for my boat?” Good question. Deep cycle marine batteries come in several sizes, known as groups, with sizes 24, 27, and 31 most common to boating applications. Note that group size has nothing to do with that battery’s storage capacity, it simply relates to the physical dimensions. On most vessels, the battery compartment will fit one of these specific sizes. Generally, you’ll want to select the largest size that fits in your battery compartment.

Cardinal Rules

“No matter it’s size or type, happiness for any marine battery is a full recharge after every use,” says Holst. “That’s a best practice you need to follow. Never allow an FLA battery to discharge below 50-percent. And never leave full discharged lithium-ion batteries discharged for an extended time – doing so is one of the few ways to kill them.

You’ll want to add a battery monitor or manufacturer’s app to protect your investment. Norsk Lithium, offers a free app for Android and IOS that allows owners to monitor all critical battery parameters including State of Charge (SOC), Amps In/Out, Estimated Runtime @ Current Draw, Charge Cycles, Cell temps, etc. It may also be necessary to upgrade your inverter and battery charger to maximize performance.

Should you find yourself on the water with a belligerent battery, Sea Tow is ready to help, 24/7. For some simple steps that can quickly get you back up and running, check out our blog: Dead Battery? Here’s What To Do. If you still need additional help, we’re just a phone call away.


Sea Tow has been the premier leader in on-water boating assistance since 1983. With Sea Tow, you pay one annual rate and receive the most comprehensive member benefits available. Membership includes a full suite of on-water assistance services for every boat you own or rent. Learn more about our membership options today. To find the Sea Tow closest to you, use our service locator or call our Customer Care Team at 800-4-SEATOW.

Tom Schlichter

Tom is a full-time outdoors writer, editor and marketeer living on Long Island, NY. Follow him on Facebook at @outdoortomcorp or visit his website at

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