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How to Charge a Boat Battery on the Water: Detailed Process

Fact checked by William Hart

how to charge a boat battery on the water

Being able to trickle charge marine battery will upgrade your fishing game. You can go further on the water and stay for long periods. If the kids want to watch a movie, they can charge their tablets. Your livewell, ice maker, and fridge will operate perfectly.

So, how do you get power supplies when you are on a long fishing trip? How to charge a boat battery on the water?

To charge trolling motor batteries while on lake, you have two options: onboard alternator chargers and solar panel systems. Though they cost more than regular chargers, both are easy to install, maintain, and operate.

Batteries can’t help unless they are fully charged. Below, you will find a step-by-step guide on each method.

Table of Contents

How to Charge With an Alternator

What you need

Installing an alternator will take you less than an hour. You only need an onboard alternator charger, its installation kit, and the owner’s manual.

Steps to charge a boat battery with an alternator

Alternators use the amperage your engine produce when it’s working to charge the batteries. They charge the battery while driving. But don’t mistake them for onboard marine battery chargers, which require a 120V wall outlet to operate.

Step 1. Prepare the battery to charge


Before installing the device, make sure your battery is in good condition:

  • Use a cloth to clean up any dirt and grime on the battery.
  • Clean the terminals.
  • Add enough distilled water to each battery cell. Do not overfill and remember to replace the caps properly. You should check and refill the battery fluid level every 30 days.

If you’re charging a boat with 2 batteries, carry out this step for both.

Step 2. Mount the alternator

You should find a nice spot to mount the alternator charger. Avoid:

  • Mounting it directly above or under the batteries.
  • Mounting it below the waterline or close to fuel tanks.

On the back of the alternator, you will find four mounting holes. When you have the location, use the bolts, nuts, and washers in the installation kit to secure the device.

Step 3. Run the wire

Different alternators require different wiring procedures, so refer to your owner’s manual to set up the cables. In most alternators, you have four wires to run:

  • The input starting cable goes to the starting battery.
  • The output cable goes to the trolling battery.
  • Additional output cables go to other batteries you have onboard.
  • The ignition wire connects to the Ignition Circuit.

Step 4. Operate


Now, when you turn on the ignition of the boat, the alternator will automatically trickle charge your batteries. When they are full, the alternator will stop charging. So, you don’t have to worry about how long to charge a boat battery.

Pay attention to maintaining the alternator charger. You should check the device and its terminals regularly and clean up any dirt, oil, and corrosion.

As long as the engine is running, the alternator will keep boat batteries charged on the water. But if you intend to be at anchor for a long time or use your vessel as a houseboat, take a look at the next option.

How to Charge a Boat Battery With Solar Panels

What you need

  • Solar panel
  • Solar panel installation kit
  • Wiring kit
  • User’s manual
  • Battery case (optional)

You don’t need fancy solar panel systems to keep your boat charging. If you own a small Jon boat, a mono solar panel is enough to charge the batteries.

Steps to charge a boat battery with solar panels

Step 1: Prepare the batteries

Before charging, always check the batteries. Since they contain sulfuric acid and might produce explosive gases, make sure the area around them is well-ventilated and dry.

Remove any dirt or grime on the batteries and terminals with a cloth. If you spot any corrosion, use a mixture of baking soda and water to clean it up. Then, refill the distilled water in each cell.

Step 2. Wire the battery

Now, put your battery in the case. The case will protect it from the harsh marine environment and eliminate incidents. If you choose not to use the case, place the battery in a safe and secluded area.

After that, open the installation kit and wire the battery:

  • Connect the motor cable to the battery. The black wire goes to the negative terminal and the red wire goes to the positive terminal.
  • Hold the Battery Status wire and the Solar Panel Side Connection wire together and place them on top of the motor cable.
  • Connect the Solar Panel Charging wire to the battery’s input cable.

The video below will help you with this step:

Your solar panels might need a different wiring procedure, so always refer to your owner’s manual.

Step 3. Install the solar panels


Start by finding a good spot for the solar panels on your boat. The spot should allow the panels to soak in sunlight at all times. You can mount them on top of your vessel or place them openly on the deck.

On smaller boats that don’t come with a roof or large deck space, we recommend placing the panel on a J-arm handle and hooking the handle to the side of your vessel.

Step 4. Finish up


Finally, run the wire from the panels to the batteries. You should use zip ties to keep the cables in place and trim off any redundant pieces for a clean look. In addition, close the cap of the battery case and tighten it firmly.

Solar panels will provide a steady supply of power for your batteries while connected due to their unlimited resources. Charging a boat battery doesn’t require a large amount of amperage, so a mono solar panel is sufficient. This option is sustainable and environmentally-friendly.

Safety Precautions

Alternators and solar systems are powerful charging devices, and charging a marine battery on the water is a risky procedure. Thus, always study manufacturers’ specific precautions before installing any battery charging system.

  • If you accidentally contact battery acid, immediately rinse it off with soap and water.
  • You should remove any metal accessories (e.g., rings, bracelets, watches, necklaces) when working with batteries.
  • You must connect the charger outputs to the batteries with the correct polarity.
  • You must never touch the terminal rings when the charger is operating.


You can enjoy longer and more exciting trips when you know how to charge a boat battery on the water. Now, you will never have to worry about power supplies for your livewell, freezer, and cellphone.

So, alternator vs solar panel, which one suits you? When will you execute the installation? Tell us in the comment section.

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