Choosing a Marine Battery | Sea Tow

23, Jun. 2025

 

Choosing a Marine Battery | Sea Tow

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.

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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 call away.

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Marine Battery Guide: Best Practices for Types, Charging, Wiring ...

Whether you're a seasoned boater or planning your maiden voyage, understanding your marine battery is essential to boating. A reliable marine battery is the heart of your boat's electrical system, providing the power you need to start the engine, run electronics, and keep everything functioning while you're on the water.

Boat Maintenance Guide

What Is A Marine Battery?

Marine batteries are specially designed to handle the unique challenges of the marine environment. They power everything from your boat's engine to electronics and accessories. 

There are three main types of marine batteries, each serving a different purpose:

Starting (Cranking) Batteries

These deliver quick bursts of energy to start the boat's engine. They are not designed for extended, continuous use.

Marine Deep-Cycle Batteries

A deep-cycle battery is built to provide consistent power over a long period, and it is ideal for running electronics, trolling motors, and appliances.

Dual-Purpose Batteries

As the name suggests, dual-purpose batteries combine the cranking power of a starting battery with the deep-cycle capability, though they are less efficient than specialized batteries.

Are you interested in learning more about marine battery pack? Contact us today to secure an expert consultation!

Deep-Cycle vs. Cranking Batteries: What's the Difference?

A common point of confusion is the difference between deep-cycle and cranking batteries.

Cranking (Starting) Batteries are designed to quickly deliver a large amount of power to start your engine. Once the engine runs, the alternator takes over, and the battery isn't needed for long-term power.

Deep-Cycle Batteries are engineered for steady, sustained energy output, which is ideal for electronics like fish finders, GPS systems, and onboard appliances that require a continuous power supply over long periods.

If your boat requires both functions, consider a dual-purpose battery, although it won't perform each task as well as the specialized versions.

Read Next: How to Winterize Your Boat

How to Choose the Right Marine Battery for Your Needs

Choosing the right battery depends on your boat's requirements and how you use it. Here are a few factors to consider:

  1. Boat Size and Power Needs: A small boat with minimal electronics will have different power needs than a large vessel with sophisticated systems.
  2. Battery Types: If your boat demands high bursts of power for starting and long periods of power for running electronics, you may need separate starting and deep-cycle batteries or a dual-purpose battery.
  3. Reserve Capacity: Look for a battery with enough reserve capacity (RC) to power your systems if your alternator fails. RC measures how long a battery can provide 25 amps of power before dropping below 10.5 volts.
  4. Cold Cranking Amps (CCA) and Marine Cranking Amps (MCA): These ratings indicate how well a battery can crank an engine in low-temperature conditions.

Understanding Marine Battery Numbers and Ratings

When selecting a marine battery, you'll come across various numbers that represent its performance:

  • Cold Cranking Amps (CCA) vs. Marine Cranking Amps (MCA): Again, CCA refers to how many amps a battery can deliver in cold temperatures, while MCA refers to its cranking power in marine environments, which are usually warmer. Most boaters should focus on MCA.
  • Amp-Hours (Ah): This number measures the battery's capacity. It tells you how long the battery can deliver a certain amount of current. The higher the Ah rating, the longer the battery can provide power.
  • Reserve Capacity (RC): RC is how long the battery can supply power before it needs to be recharged. It's a critical number to look at if you're running electronics off the battery.

Charging a Marine Battery: Best Practices

The environment is hard on your marine battery - everyday heat can considerably shorten battery life in warmer climates. In areas with cold weather that calls for winter storage, a battery that is not kept charged can freeze in colder temperatures. 

Proper charging is essential for extending the life of your marine battery. Here's how to keep your battery in peak condition:

  • Only use a charger designed for marine batteries. Overcharging or undercharging can shorten battery life.
  • Different types of batteries require different charging voltages. For example, a 12-volt deep-cycle battery should be charged to around 14.4-14.8 volts.
  • Consider using a trickle charger to maintain battery health over long periods when your boat is not in use. It's best to keep batteries on a regulated "trickle" charger to hold a charge while not in use. 

How to Maintain and Extend the Life of Your Marine Battery

Here are some Tips for Avoiding Battery Problems and how to keep your battery in excellent condition:

  • Frequently check the battery terminal connections to ensure they are snug and corrosion-free. Check for cracks, bulges, or leaks. Any signs of damage should be corrected as soon as possible.
  • Corrosion around the terminals can prevent your battery from functioning correctly. Clean the terminals regularly using a mixture of baking soda and water.
  • High temperatures can shorten the lifespan of your battery. Keep it cool and out of direct sunlight when possible.
  • Vibration can damage your battery. Secure the marine battery with a good battery box or battery tray, which should have a base that is screwed or bolted to the boat and either a rigid bracket or a locking strap to hold it to the base. You don't want the battery banging around in rough water.
  • If you use the boat infrequently, use a maintenance-type battery charger to keep the battery fully charged between outings.
  • Before off-season storage, completely charge the batteries, then disconnect the terminals so nothing can draw the battery down. If there's power available at your storage site, keep the batteries on a battery maintainer/charger through the off-season to continually maintain your batteries. Otherwise, remove the batteries from the boat and store them where they can be connected to a maintenance charger.
  • Install a cover or "boot" over the top of the positive battery terminal if one was not installed by the boat builder, even if the battery is in a covered box. The boot prevents sparks and arcing and possible explosion if, for instance, a tool is dropped on the terminal.

Storing Marine Batteries During the Off-Season

If you don't use your boat year-round, storing your battery correctly during the off-season is essential. Always fully charge your battery before storing it. A partially charged battery is more likely to freeze. Store your battery in a cool location to prevent overheating or freezing.

Check the battery's voltage regularly during storage and recharge as necessary to prevent deep discharge.

Troubleshooting Common Marine Battery Issues

If you run into problems, here are some common issues to watch out for:

Battery Won't Hold Charge

This could be due to sulfation, where lead sulfate crystals build up and reduce capacity. Regularly charging your battery can help prevent this.

Rapid Drain

If your battery drains quickly, you may have a parasitic load, where some device is drawing power even when the boat is off.

Signs of Aging

As batteries age, they hold less charge. If your battery is several years old, it may need replacing.

When and How to Replace Your Marine Battery

Marine batteries typically last 3-5 years, but this can vary depending on use and maintenance. Common signs that it's time for a replacement include:

  • Slow Cranking: If your engine takes longer than usual to start, it may be time for a new battery.
  • Swelling or Leaking: Any physical damage is a sign that replacement is necessary.
  • Reduced Capacity: If the battery doesn't hold charge like it used to, it's likely reaching the end of its life.

Consult your boat owner's manual or a marine dealer when replacing a marine battery. 

Buy a new marine battery that is a good match for your boat. When shopping for a deep cycle battery, you'll want to pay the most attention to the ampere-hour rating and reserve capacity. 

For starting batteries, focus primarily on the marine cranking amps. Consult all three rankings when searching for a dual-purpose battery.

Suppose you add electrical accessories to your boat. In that case, you may need to upgrade to a battery with a higher amp-hour rating, especially if you spend a lot of time trolling with the engine at a very low speed, beached, or at anchor while using accessories like the audio system.

Dispose of your old battery responsibly by taking it to a recycling center or returning it to a marine supply store.

Marine Batteries

Choosing the correct marine battery and maintaining it properly can make all the difference when you're out on the water. Knowing how to select, care for, and replace your battery will mean you enjoy smooth sailing all season long.

By following these tips, you'll maximize the lifespan of your marine battery and avoid unexpected issues so you can focus on what matters most – enjoying your time on the water.

Looking for more on basic boat maintenance? Read...

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