How To Charge A Boat Battery On The Water: The Complete Guide

While charging the battery in port should not be a problem, charging battery, or more often, batteries, on the water for many skippers can be a problem for many reasons.

Having discharged battery can spoil any fishing, leisure, or similar boat trip. However, boat batteries can be easily charged using several methods, and with some care, they can be kept fully or almost fully charged most of the time.

Published: March 13, 2023.

Short Intro To Boat Batteries

Boat batteries range from small Powersports batteries intended for starting small gasoline two- and four-stroke outboard engines to large battery packs intended for cranking huge diesel engines and for powering bow and stern thrusters.

Also, for powering onboard electronics (for example, fish finders), one may use small 12V lead-acid and lithium batteries, while for powering electric trolling motors and boat electric motors, one may use large 48+ volts battery packs.

And best of all, boat electric systems can be one- and two-battery (or battery packs) electric systems, with one battery intended for cranking and another battery intended for deep cycle applications, with each battery having somewhat different charging requirements.

So, how to charge boat batteries while not being at the port?

First of all, if You have two battery electric system, we must assume that You also have a good and reliable DC-to-DC battery charger, like Renogy 12V 20, 40, or 60 Amps (Amazon link, opens in the new window) that separates/isolates cranking/starting battery with charger controller and deep cycle battery.

Also, another "must-have" is a good onboard charge controller that allows the user to charge the batteries using various methods.

Main Engine

Except for a small percentage of electric-only boats, all boats feature internal combustion engines powered by gasoline (gas) or diesel fuel. Just like car engines, boat engines also feature alternators that produce electric energy, which is then sent to the charge controllers, which then charge the batteries and power the rest of the boats' electric system.

So, while the boat is on the water and away from the port, as long as the main engine is running, the battery (or batteries) are being charged.

However, there are situations where the main engine can't be used to charge the batteries for various reasons. In that case, there are other methods for charging onboard batteries.

Solar Panels

Solar panels collect sunlight and convert it into electric energy, which is then sent to the charge controller, which obviously must have solar panel input.

Solar panels can be rigid or flexible and can be placed in a permanent location on the boat or in a temporary location.

Solar panels have many benefits and a few cons, including:

• As long as there is sunlight, they produce electric energy,
• The more sunlight they collect, the more electric energy they produce,
• Solar panels are relatively lightweight and can be mounted even on uneven surfaces,
• During the night, solar panels don't produce electric energy,
• Solar panels are quiet and fully passive, requiring no fuel other than the sunlight,
• When the boat is in the port but not connected to the port's electric installations, solar panels can be used to keep the batteries fully charged, etc.

Due to all these benefits, solar panels are the number one alternative power source for many boats and yachts.

For example, if there are 500W solar panels mounted on the boat, and due to various reasons, their efficiency is "only" 50%, then these solar panels deliver 250W to the charge controller. If the charge controller's energy efficiency is 90%, then the charge controller delivers 225W to the battery, assuming that all the energy is used for charging the batteries.

If the charging voltage is 14.4V, then the charging current is ~15.6 Amps, allowing such solar panels to recharge 12V 100Ah lithium battery discharged down to 80% DoD in 5.5-6 hours.

Wind Turbines

Wind turbines are not as popular as solar panels for several reasons, especially due to the noise, more mechanically complicated setup, and the fact that solar panels do their job so well.

But, as long as there is wind, even during the night, wind turbines can recharge the boat battery.

For boat owners that don't want to turn their engines On often and who don't want to rely exclusively on solar panels, wind turbines are choice number two.

Portable Power Generator

Small portable inverter power generators can be used to charge the onboard batteries without starting the main engines On.

However, one must be aware that power generators also feature internal combustion engines, which are noisy, and that they emit smelly and toxic fumes that contain Carbon Monoxide (CO) - if a portable power generator is going to be used on the boat, it must be kept outside, downwind from people, pets, windows, etc.

Some of the most popular portable inverter generators are 2000W Power Generators that feature 120V AC receptacles, but also they often come with a 12V DC charging ports.

Personally, if You are going to fire up an inverter generator to recharge your boat battery, use a 120V AC port and fast advanced battery charger and recharge your batteries ASAP.

Again, power generators can be noisy, and they emit dangerous Carbon Monoxide.

Portable Power Station

Portable power stations, also known as solar generators, feature solar panel charging ports, allowing the users to charge power stations' internal batteries directly.

Also, they feature 120V AC receptacles and sometimes 12V charging ports. So, in theory, one can use a power station/solar generator to charge the boat's onboard battery.

The main problem is that the power stations' batteries store less energy required for the full charging of a typical boat battery, and in most cases, solar generators can't be used to fully recharge fully discharged boat battery.

However ...

Many models of power stations support so-called pass-through charging - the power station is connected to the solar panels (used to charge the power station), and at the same time, the power station is used to charge the boat battery.

In such situations, the power station can fully recharge the boat battery without draining its battery completely.

Note: unlike power generators, power stations can be used indoors.

Hand-Cranked Power Generator

Hand-cranked generators are not very often used for charging the main boat battery for a very simple reason - it would be a very time-consuming task to recharge, let's say, 12V 100Ah battery with a hand crank power generator providing 2-3 Amps @13-14V continuously.

But, such hand-cranked power generators can be used to recharge battery jump starters, power banks, smartphones, and similar devices, which can be a game changer in many situations.

Few Final Words

 As one can see, there are many ways of charging the boat battery while on the water.

The most common one is using the boat's main engine and, when it is turned off, using solar panels.

Other methods may also be used, especially wind turbines, but they are not that common.

Hand-cranked generators are used almost exclusively in emergencies to charge telecommunication devices and battery jump starters, which can help start the engine while the main starting battery is discharged.