Boat Batteries: Everything You Need To Know

Every boat that has an engine with an electric start will have a battery, and that battery has some unique things about it that every boat owners needs to know about.

While a boat and a car use a lead-acid battery, they’re not completely the same, and the way you take care of a boat battery will be different.

In this post, I want to cover all the major points about boat batteries and the charging systems on boats.

Boat Batteries Explained

A boat battery tends to be larger than most car and truck batteries, as starting a boat engine can require more power.

For all modern boats, they use a 12-volt battery, and some larger diesel-powered boats will have a 24-volt battery. If you’re not certain, the old battery will clearly state its voltage or your owner’s manual will also tell you.

These batteries are lead-acid, which makes them heavy and suffer from sulfation that we will cover later. Lithium 12-volt batteries are still new and not something you see standard for many manufacturers or dealerships. If you do have a lithium battery, you need a special battery charger made for charging lithium batteries.

What Boat Batteries Do

The battery in your boat does two main things, start your engine and have a pool of power for electrical devices on your boat.

It requires a lot of power to start an engine, it’s not uncommon for a boat to pull 30 amps or more, which is twice the power that comes out of your home’s standard wall outlet. It’s only for a few seconds that the power is needed, but it’s very important to have, and that is why some boats will have two batteries.

For dual boat batteries, you have a starting battery and a deep cycle battery. The starting battery is used to start and run the boat, but the deep cycle is for when you’re sitting with the engine off and need to run things like the radio.

Modern boats use a fuel injection system, and these systems use power from the battery to keep the engine running. A bad battery can cause all kinds of running issues, so it’s important you have a good and charged battery for your boat.

Deep Cycle Explained

You can get boats that offer dual battery systems, some can even have more than two depending on your needs.

The reason for a deep cycle battery is to run electrical items off it instead of your starting battery. If you drain your starting battery because you have been running the radio all day, you won’t be able to start your boat.

A deep cycle battery can be drained much further than a starting battery and supply a more constant load. A starting battery is best for starting the engine and being a little tank of energy to power the must-haves of your boat’s electrical system.

A battery switch exists that allows you to swap between the two batteries or even run both at the same time. You would only run both batteries together as a last ditch effort in starting a boat, as both together will have more power. You don’t want to charge a starting battery and deep cycle battery together, as they charge at different rates.


All boats have some type of charging system to keep the battery charged and the items that need electrical power running too.

For smaller boat engines, like outboards, jet skis, jet boats, and fishing boats will use a stator. Larger boats or inboard boats will tend to have an alternator, mostly because these engines are modified SUV or Truck engines.

A stator and alternator work very similar to each other, but an alternator is better.

A boat will charge while idling, but it’s best to use a battery charger if the batteries are weak.

Chargers – Needed Or Not?

All lead-acid batteries need to be charged for them to work for your boat.

Driving your boat will charge the battery, but if the battery has been sitting and the boat not run for a while, the battery needs to be charged by a battery charger.

Some boats come with onboard marine battery chargers and when the book is hooked up at the dock to the power source the batteries get charged too. Not every boat comes with onboard chargers, and it’s up to you to keep them charged.

How Often to Charge

A boat battery will lose it charge, or better yet, cranking power, if you leave it alone and don’t drive your boat for more than a month.

Lead acid battery sulfate when not being used, and it kills batteries, and they lose their charge. When you charge a boat battery, you’re removing the sulfation that builds up and bringing the battery back to life.


It can take a month or several months for a battery to sulfate and lose its charge. A sulfated battery can be charged and come back to life most of the time. A battery that’s been sitting for years more than likely will not come back.

You should charge your boat battery at least the night before you take it out if you have not driven it for months.

If your boat gets some sun, I suggest getting a solar battery charger for your boat.

For more information on charging boat batteries, check out our post here.

What Will Happen If You Don’t Keep Your Boat Battery Charged?

Ideally, you should charge your boat battery every 30 days or keep it on charge when not using it, but it’s not the end of the world if you don’t.

It’s all about the luck of the draw.

I have people who never charge their boat batteries during the winter months and the boat starts just fine.

I then have far more people who don’t charge their boat batteries, and they’re dead at the start of the new season, and they have to buy a new battery.

Since I have more people whose batteries die when not being used for months, I tell everyone to charge their batteries every 30 days or keep it on charge with a smart battery charger or proper solar charger. Batteries are expensive and annoying to replace, but charging a battery is often cheaper and way easier.

Disconnect Battery From Your Boat When Charging

When charging your boat battery, it’s ideal to disconnect it from your boat or turn the battery switch to off.

If you forget to turn the boat off before charging, it’s not always a huge deal, especially for modern boats. At worst, you may blow a fuse and the very rare cases fry the computer.

When using a battery charger stick to the ones that are lower amps, 2 to 5 is fine, even 10 is fine for large boat batteries. The more amps the charger has, the quicker it charges a battery, but more things can go wrong if you’re not careful.

How Many Amps To Charge Boat Batteries?

The amps of a battery charger determine how long it will take to charge your boat battery.

The more amps the charger can supply, the quicker the battery will charge, but there are some “ifs” to that.

Low & Slow

Ideally, it’s better to charge your boat battery low and slow than high and fast. Fast charging boat batteries can damage them and sometimes give you a fake charge. Low and slow means the battery will take longer to charge, but be more worth it in the long run.

Charging Boat Batteries – Time Needed

For best results, you should charge your boat battery overnight on a low amp (2 to 5 amps) smart battery charger.

Depending on the battery charger, a fully charged lead acid battery can be ready in 4 to 8 hours.

You never want to rush a lead acid battery when charging it as it could lead to a fake charge, it may start the boat a few times but give up on the 3rd. Slow and long charging is the best way, and that means charging your boat battery overnight. You must also use a smart battery charger, basically one that turns off when done charging.

That is why I tell people to charge their boat battery the night before they go out, just to be ready.

Sealed Batteries Are Charged From Factory

When you buy a new boat battery, and you don’t have to fill it up with the acid, then it should be charged and ready to go.

Before leaving the store, have them load test the battery. Do not use a volt-meter, you need a load tester, as volts can lie to you.

If you have the time, it doesn’t hurt to charge the boat battery with your smart charger. You don’t know how long that battery has been sitting on the shelf, or even may have been used as a jump battery by the dealership or store.

What To Do If The Boat Battery Won’t Charge?

A boat battery that won’t charge can sometimes be fixed, but a lot of the times getting a new battery is the only answer.

If the boat battery does not charge, then make sure you’ve connected correctly to the battery charger.

Do not rely on your boat to charge the battery, get an actual battery charger. Many boat dealership may even do it for free on their large battery banks and that bring it back to life.

The biggest reason your boat battery is not charging is that it’s sulfated, and using a desulfator smart battery charger (ad) can help bring it back to life. The desulfation may not work for all batteries, but well worth trying on the batteries that can handle it.

Storing For The Winter

If you’re not going to be using your boat for months, often during the winter, it’s ideal to take the battery out of your boat if you keep your boat out of the water.

If your boat is kept on the water, even on a lift, I would keep the battery in it, so the bilge pump can still work. I would prefer to hook up a solar battery charger instead, to make sure the battery stays good for the bilge pump.

Keep Off Ground

When storing your battery, it’s best to keep it off the ground to insulate it from the cold as much as possible. The cold speeds up sulfation and will kill your battery. Ideally, keep a smart battery charger (ad) on your boat battery, as it will turn on and off as it needs it.

Please keep in mind when storing your boat on land during the winter to keep a cover on it, ideally a shrink-wrap one if you get snow where you live and the drain plug out. Tilt the trailer back a little to let the water drain out, even with a cover on water tends to find a way in, so you want to get it out as it freezes and makes things worse.

Bilge Pump Still Works When Battery Is OFF – If Wired Correctly

When you turn your boat off at the battery switch, the bilge pump should still work if wired properly. I say this as some people install aftermarket ones and do it wrong.

Since the bilge pump still works with the boat battery set to off, it’s ideal you keep the boat battery charged when not using it, especially if kept in or above the water. If you keep your boat on the trailer or on land for storage, you should take the drain plug out, so the water can get out that way.

Make sure you have the drain plug in your boat before you go in the water! If you have a pontoon, you may or may not have a drain plug.

Will A Boat Start Or Run Without A Battery?

Unless your boat has a pull start, it will need the battery to start and run.

Some carbureted boat engines can run without the battery, but may need it to start the engine.

Modern boats use a fuel injection system, and they need a constant and reliable pool of power to pull from, and the stator or alternator can’t handle the spikes of power. The battery on your boat acts as a buffer for these electrical components on your boat and is important to have!

How Long Do Boat Batteries Last?

Boat batteries can last 3 to 5 years, but not uncommon to get more years out of them.

For the full break down and how to get the most life out of your battery, check out his post here.


You can jump start a boat battery from another battery that is not connected to anything, for example, a jump starter pack.

What is not a good idea is to jump start your boat with your truck or other running boat.

Even though they all use 12-volt systems, they’re all not rated at the same amp load, and you could either fry your truck or your boat’s electronics system. Also, boat batteries are much larger and your truck seeing that big deep load being put on it can stress it so much that a check engine light can come on as that battery is not normal, and it’s having a hard time adjusting.

When it comes to jump-starting a boat, I carry a super capacitor jump starter. (ad) You don’t need to keep it charged, can charge off a dead battery, and can sit doing nothing for years and still work just fine. It’s border-line magic, but it’s real and works, and it’s got me out of a few pickles before.

The Order To Connect & Disconnect Batteries

When adding a battery to your boat, you want to do the positive cable first, then the negative.

When removing the boat battery, you do the negative first and then the positive.

You should have your boat’s battery switch set to off when doing any electrical work.