How to Jump a Boat Battery – Boat Battery Basics

Published Categorized as Maintenance and Fixing Guides

Boat power systems vary greatly depending on how the owner uses the boat. Therefore, it is tough to figure out your boat’s system as a new boater. However, taking a look at your entire system will give you a leg up should the day come when you need to figure out how to charge a boat battery on the water.

Understanding the system is critical to your enjoyment of the boat. Knowing what to do when the boat battery is dead, how to charge it, and figuring out why it died will save the day should you need to jump a boat battery one day.

Table of Contents

red and black coated wire

Battery Basics and Troubleshooting

At their most basic level, boat electrical systems are very similar to your car’s. The simpler your boat is, the more like a car or truck it will be. 

If your boat has an electric starter, chances are it has an engine alternator. So just like a car, once the engine is running, the motor will also charge the battery back up. 

One crucial difference is that some boats use different voltages than cars. While most of them use 12-volt systems with traditional 12-volt batteries, some use 24 or even 48-volt power systems. As a result, you cannot simply jump a boat battery with a different voltage. 

An invaluable tool in troubleshooting any electrical problem is a portable multimeter. Having one of these in your toolkit will enable you to test battery voltages to see if you indeed have a dead boat battery. They will also enable you to test electrical circuits, which can be a great help in understanding the battery switches and how your boat is wired.

Uni-T B4Q094 UT210E True RMS AC/DC Current Mini Clamp M W Capacitance Tester

Cruising Boat Battery Systems

The most significant difference between cars and boats is that a boat uses the battery’s electricity for more items. For example, a fishing boat might turn off its motor and fish for a while. While the motor is off, the crew might run a few baitwell water pumps, the stereo, chartplotter, electronics, and the boat’s running lights. All of that stuff is draining the battery. If the crew isn’t careful, they might not be able to start the engine when they’re done fishing.

The solution for most boats is to keep two batteries onboard. One battery is solely for the engine—it starts it and charges when the motor is running. This is known as the starting battery. 

When the boat isn’t running, the other battery is being used for all of the power needs. It is called the “house” or “domestic” battery. Ideally, if the crew drains that battery, the starting battery will still work and start the engine. The alternator will then work to charge both batteries with the engine running. 

House batteries are usually designed differently than starting batteries. They generally are deep-cycle batteries, designed to take deeper depths of discharge than starting batteries are. 

The Art of Power Management

As you might imagine, things can get complicated with two different types of batteries on board, each with its own purpose. 

When you start looking at liveaboard cruising boats, the house battery is more likely to be a house battery bank made up of several individual batteries. These boats are designed to run for a few days away from the dock and without the motor running.

How the two batteries or battery banks are separated varies from boat to boat. For decades the standard method was with a simple battery switch. The switch could select OFF, I, II, or BOTH. 

Seachoice 11501 Battery Selector Switch, Red, One size

Turning the battery switch OFF kills all power to everything. It is imperative never to select OFF when the engine is running. Many owners select OFF when they leave the boat in storage, although this might kill power to essential systems–like the bilge pump–on some boats.

Turning the switch to either I or II selects that battery and nothing else. So, for example, if Battery I was the house bank, it could be selected while sitting at anchor or fishing.

Selecting BOTH meant that the two battery banks were working in parallel, so power was coming from both, and the alternator was charging both. This is the standard setting for running, but if you left the switch in BOTH all the time, it would eventually drain power from both batteries.

If this all seems to you to be complicated and easy to mess up, you are correct. Thankfully, the automatic charging relay (ACR) is a more modern equivalent. The ACR does all the work for you – it senses when the engine is running and charges both, and it isolates the starting battery when the engine is not running.

All of this provides a simple solution if you are jumping a boat battery. Usually, the starting battery should be isolated from the house bank. But if you find the starting boat battery dead, you can usually use the house bank as a spare battery for jump starting a boat.

Different Marine Battery Chemistries

It’s also vital to realize that not all boats use standard lead-acid (LA) batteries. Instead, many variations of LA technology are used on boats and some completely different options.

The traditional lead-acid car battery is known as a flooded lead-acid battery. This is because the battery plates are submerged in an electrolyte that you must occasionally replenish with distilled water. Some of these batteries are sealed and dubbed “maintenance-free.” This is still the most common type of battery used as a starting battery. 

The liquid electrolyte model has variations, including the Gel and AGM (absorbed glass mat) batteries. These are similar to flooded-cell batteries, but they do not require any maintenance.

There are also unique formulations and modifications of the basic LA, like the Firefly batteries from Oasis. This type of marine battery can be more deeply discharged than others.

These different chemistries aim to solve some of the most fundamental problems of batteries – they have a limited lifespan that gets shorter and shorter the deeper you discharge them. For the most part, lead-acid batteries do not last very long if they are allowed to be discharged below 50 percent regularly.

Some new lithium technologies are being used in boats. The most common lithium battery being used is the lithium iron phosphate battery, commonly referred to as an LFP or LiFePO4 battery. These batteries last much longer and can be discharged very low, sometimes down to 90 percent deep discharge or less. But they also have very particular charging profiles and require a more complex installation and system to work safely. 

Each battery installed on a boat will come with limitations. The more complex the battery, the more critical it is to follow those limitations to the letter. Look to the battery manufacturer to provide a charging profile and installation instructions.

The choice of which battery chemistry is right for you is beyond the scope of this article. The point, instead, is to emphasize that boat batteries come in a wide variety of shapes, sizes, and chemistries. So before you jump a boat battery, make sure you understand the type of battery you are working with and its requirements. 

And one final word of caution—according to BoatUS insurance statistics, one of the leading causes of boat fires is improperly wired batteries. Batteries contain a vast amount of potential energy, and if shorted or wired improperly, they can explode. So be extra cautious that all wiring is correctly installed and that you understand how your system’s selector switch works. 

Types of Battery Charging on Cruising Boats

Any boat with a motor probably uses an engine alternator to charge the batteries. If there is a starting battery, the alternator will recharge it. If there is also a house battery, the alternator will usually recharge it. 

However, large house banks often have many other ways to be charged. Here’s a list of a few ways that you can charge boat batteries.

  • Engine-mounted alternator or generator
  • Stand-alone generator
  • Shore-power battery charger
  • Renewable sources – solar panels, wind, or hydro generators

How to Charge a Boat Battery on the Water

So you’ve turned the key, and nothing happens – it looks like you’ve got a dead boat battery. There are basically four ways to jump start a boat. Whether your boat has an outboard motor or inboard, the procedure is the same.

Method 1. How to Jumpstart a Boat with Two Batteries

If a boat has a traditional battery switch or a modern ACR relay, you should be able to start the engine with the house battery. Turn the switch to BOTH and see if the engine will start. If not, both batteries may be dead.

Method 2. Jumping a Boat Battery from Another Battery or a Car

Can you jump start a boat just like a car’s battery? If the boat has the same voltage as your other vehicles, usually 12 volts, then the answer is yes. You just have to get the car close enough to the boat for the jumper wires to reach. You could also trying using any spare battery or backup battery, so long as it is fully charged.

If you have a good battery, you can use jumper wires and jump the boat just like you would a dead car. Attach the cables correctly, first the black to the dead battery’s negative terminal, and then the red to the dead battery’s positive terminal. Then repeat the process on the good battery. Once the batteries are linked together, you can start the other vehicle’s engine and charge your dead battery for a few minutes. 

If you’re familiar with jump starting cars, you know that the frame is used as the negative or ground connector. This is not the case in boats, so make sure that the negative cable is secured to the negative terminal on the battery. All negative electrical connections on a boat come back directly to the negative terminal.

Once you’ve connected the cables, the voltage will hopefully climb enough in your battery that you can try to start your engine. If your engine runs, you can disconnect the batteries and go on your way. Use caution to avoid sparking when you disconnect the jumper cables!

person holding red and black hand tool

Method 3. How to Use a Boat Battery Jump Starter

A more straightforward solution is to use a portable battery jump starter. These are small rechargeable battery packs that store enough electricity to get you going when you have a dead battery. The starter will have jumper cables coming off that attach to your dead battery. Clip them on correctly, with the red one on the dead battery’s positive terminal and the black one on the dead battery’s negative terminal. Then turn the charger on. You can then try to start the boat’s motor.

Method 4. Use a Corded Battery Charger

If all else fails, you’ll have to get the battery a proper charge. If the boat is equipped with a built-in battery charger, you should be able to plug the shore power cord in and let it sit. Depending on the size of your charge, it might take a few hours. 

If your boat doesn’t have its own battery charger, you can remove the battery from the boat and take it home to charge. Portable battery chargers are inexpensive. Just make sure you disconnect it once the battery is topped off. 

What if the Boat Battery Won’t Charge?

If the battery still doesn’t start the boat after a proper charge, you probably need a new one. Take it to your local auto parts store, where they can plug it into a load tester. It is possible the battery is showing charged voltage but no longer has the capacity to start your engine. 

Boat Battery Charged, But No Power

What if you have charged the battery, but the engine still isn’t starting? You can usually confirm the state of charge with your multimeter. Place the probes (red on positive and black on negative) and set the dial to DC voltage. A charged battery should read somewhere near 12 volts. If it is closer to 10 volts, it’s likely dead. 

If it seems charged, make sure the switches are selected correctly. Do any boat systems have power? Does the engine have a separate switch from the house battery system? Sometimes you just have to step back and make sure the darned thing is turned on! It’s happened to us all. 

Dead Battery? Don’t Despair!

When it comes to the basics, not much separates boat battery systems from those you find in your car. That’s good news because it means help is easy to find. You can usually find batteries that will work for a basic engine starting at your nearest NAPA or AutoZone. Plus, they can test your old battery and take it off your hands if it’s shot. You can also find wall chargers to keep your battery in top shape and charge it on your own in most stores. 

Finally, dead boat batteries are one of the many reasons you should carry marine tow insurance. In the worst case scenario where you are away from the dock and cannot start your boat, TowboatUS and Seatow will come to their member’s rescue and jump start their boats–a handy service if you find you’ve broken down away from the dock.

Boat Battery FAQs

Can you recharge a dead boat battery?

It depends on the condition and age of the battery and how deeply discharged it is. Like cars, boat batteries do not last forever and will need to be replaced every few years. Also, keep in mind that boat batteries live tougher lives than those in your car, with deeper discharges and longer periods of disuse. 

If the battery is in otherwise good condition, you can recharge or jump it. You can either jump start it with cables from a good battery or use a portable jump starter battery pack. Once the boat is running, the engine alternator should recharge the battery as usual. But if the battery does not hold a charge after that, you’ll need to do some troubleshooting and maybe replace the battery.

If you’re unsure about the condition of your battery, you can always take it to an automotive parts store and have it load tested. That will tell you if you can simply recharge it or if it needs to be replaced.

By Matt C

Matt has been boating around Florida for over 25 years in everything from small powerboats to large cruising catamarans. He currently lives aboard a 38-foot Cabo Rico sailboat with his wife Lucy and adventure dog Chelsea. Together, they cruise between winters in The Bahamas and summers in the Chesapeake Bay.