How To Charge a Marine Battery?

Having your boat’s battery run out of charge (power) and leaving you stuck in the middle of nowhere can be a very unpleasant experience. An ideal method to avoid this situation is to understand how to charge the marine battery properly.

Therefore, this article will guide you through everything you need to know about charging a marine battery. 

Starting battery, service battery, and emergency battery: what are the differences?

There are three main types of marine batteries based on their application that meet different needs and have varying technical characteristics.

Starting battery

The work of a starting battery is to start the boat’s engine in a similar manner to a starter battery on a car. This process requires the battery to supply a high peak of power in a very short time.

You can easily use a starter battery multiple times before recharging it because the engine’s alternator will start recharging the battery immediately after your boat starts.

Service battery

The purpose of a service battery is to power all the onboard equipment, such as autopilot, lights, refrigerator, etc., that run on direct current (DC).

The service battery is used throughout the entire navigation period. Therefore, it must be capable of more prolonged discharge and charge cycles than a starter battery.

Emergency battery

Also referred to as a backup battery, it is optionally used in case of an emergency, such as emergency lights, radios, etc.

Therefore, emergency batteries must have an extended life allowing them to work as backup energy reserves in case of a need.

How often should you recharge your marine battery?

We highly recommend regularly recharging your service battery during the summer months when you get back to the marina or at least when you use your boat. 

The real challenge with the charging frequency is that the batteries remain unused for long periods during winter.

A common immutable rule is that marine batteries naturally discharge if they are not used, which is referred to as self-discharge. And the battery’s self-discharge rate depends on its quality.

For instance, a standard boat battery has a monthly self-discharge rate of 5%.

Recharging a boat battery wintered on land for several months, with a battery self-discharging rate of 5% each month, can fail to restore its full capacity.

So we recommend using a battery charger during the wintered period.

Generally, the battery’s life depends on the number of charge cycles it can withstand.

And the number of charge cycles can vary significantly depending on whether you use lithium, AGM, gel, or lead-acid battery.

In any case, using a boat battery regularly will extend its life. For instance, if you recharge your battery regularly, it will last 3-4 years.

However, if you leave it inactive without recharging it during winter, it will die after one season.

Battery state of the charge reference chart

You can determine the battery state of charge by reading either the specific gravity of the electrolyte (using a hydrometer) or the terminal voltage (using a digital voltmeter).

Here is a reference chart for the battery state of charge;

Percentage of charge24-volt battery12-volt batterySpecific gravity

Charging a boat battery with a charger

You can charge a boat battery in only five straightforward steps if you have the equipped charger.

Prepare the battery carefully.

Usually, a marine battery is put in any inky place of the boat you can not easily reach.

Therefore, it is often found to have waste, corrosion and dust, which need to be cleaned before charging the battery.

Now, your marine battery is ready for charging.

Pick the right charger.

You need to pick the correct on-boat battery charger or a portable marine battery charger. 

  • Onboard battery charger: It is more expensive than a portable battery charger. And you need to install it permanently in the boat using wiring. Besides, using it is easy as it only entails plugging it in.
  • Portable battery charger: You can easily carry it wherever you like. Its cost is cheaper compared to an onboard battery charger. However, low-quality portable chargers can be troublesome to connect.

Usually, these chargers are designed with different features to serve you properly.

For example, if you own a wet-cell battery, we recommend using a quality lead-acid battery charger. It will help you charge your battery easily and faster.

Conversely, we recommend picking a smart multi-stage battery charger if you have an AGM battery. These chargers prevent your boat battery from damage risks caused by overcharging.

Connect the charger to your battery and charge.

The conventional method of charging a boat battery with a charger entails using an electrical charger with support from a 110 volts wall outlet.

Suppose you are using a portable charger; you should connect the negative and positive cables to the battery’s terminals for charging.

Next, hook the power drawing cable into a 120 volts socket.

Now switch on the power outlet, and your boat battery will start charging automatically.

However, it is vital to note that onboard battery chargers are easier to use because they are always connected to your boat’s battery system.

What you should do is connect them to a standard 120 volts power outlet.

Remove the charger

Immediately after you realize your boat battery is completely charged, do not hesitate to turn it off and disconnect the charger clips from your boat battery terminals.

Next, remember to put the top cover back into your battery box and then place your battery back in its original position.

Your boat battery is fully charged, and it is time to go boating.

The 120 volts socket of your vessel

Before charging your boat battery using a traditional charger, we recommend getting any charger that can fetch power from a 120 volts socket of your vessel.

Mains battery chargers

These chargers make it possible to charge the batteries while the boat is in port.

They also power the whole electric grid of the boat, which include refrigerators, electronics, lights, etc.

Larger boats can operate in various countries and different areas. Therefore, mains battery chargers should accept the following;

  • Various sockets
  • Different phases (3-phase or single-phase electric power)
  • Various voltages (380V, 220V, 110V)

Wind turbines

Wind turbines allow for harnessing wind energy. Generally, blades are connected to the shaft, which is connected either over or directly to the transmission box using an electric generator.

They begin to generate electric energy when there are low wind speeds. However, when there is no wind, there will be no electricity produced.

Besides, the blades can spin very fast in strong winds. Therefore, wind turbines should be carefully placed on the boat to avoid injuries and damage.

Solar panels

Solar panels convert sunlight to generate electricity. However, their input power depends on several factors, such as;

  • Type of solar panel
  • The angle of the solar panel
  • Weather
  • Season, etc.

They do not have moving parts, unlike wind turbines, and you can position them closer to the onboard equipment and passengers.

They usually function during the day, with sunlight, but produce no electricity at night.

Because of their finite resources, solar panels do not act as battery chargers. Instead, they work like power maintainers.

However, you can use solar chargers by installing the solar panel on your craft’s roof, where it will produce 12 volts DC (direct current).

Then, you need to use an inverter to convert the generated DC power to 110 volts AC.

You can then use this AC power to recharge your marine battery. However, the cost of an inverter and solar panels is relatively high.

Towed generators

They are also called hydro generators. As the name suggests, they are usually towed behind leisure or fishing boats under sails.

If the water is deep enough, we recommend submerging your turbine generator deeper (up to 15 to 20 meters) for better efficiency.

You can also use a hydro generator to generate electricity while anchored in strong sea currents, such as channels.

However, using a hydro (towed) generator is useless if you own an electric trolling motor or are using an electric boat.

Generally, towed generators are not energy efficient because you need to optimize the propeller for boat propulsion; otherwise, they can increase fuel consumption.

However, they are convenient, and you can use them to charge boat batteries automatically.

You only need to switch off the main engine and sail up with the gearbox turned to neutral.

This will generate enough electricity to power electronics and lights and charge your battery with a few amps.

Wave generator

These special types of generators are not common as they are still under development.

They are positioned behind the stern of an anchored ship or in the wake of the sailing ship.

Generally, wave generators harness wave energy. However, some V-shape wave generator models harness sea currents and wave energy.

Internal combustion engines

Generally, each internal combustion engine features a generator or alternator.

They generate energy (power) when the main engine is running. Therefore, for them to generate electricity, the main engine should run at least on idle.

Some marine boats only have a main engine paired with a generator, which generates electricity.

You can use this electricity to recharge marine batteries and run different electric motors connected to the propeller.

As such, internal combustion engines (gas, petrol, diesel) always run optimally, resulting in reduced fuel consumption.

This system is found in modern Israel subs, German subs, etc. And they are used on submarines, battleships, aircraft carriers, etc.

Portable power generators

Portable or fixed gas or diesel generators are commonly found on large or medium leisure and fishing boats when electricity is in high demand.

They have relatively low noise levels and low fuel consumption.

You can easily reposition portable generators. For example, you can transport them for maintenance, on-shore when needed, etc.

We recommend using larger models with electric starters and remote control for onboard use on larger boats.

The output power of portable and fixed generators ranges from 1-10kW or even more.

Manual power generators

They generate power for essential equipment, such as navigation lights, cells and smartphones, radios and radio beacons, etc.

They generate tens of watts of power. However, they are not ideal for charging the main marine battery.

For instance, while generating 2-3 Amps, they will need around 10 hours to charge a 12 volts boat battery with 20-30Ah.

Fuel cells

They use fuel to generate electricity with an efficiency of 70-80%.

Comparatively, the best diesel engine reaches an efficiency of 45%, while gasoline/petrol engines are even less efficient.

However, fuel cells are generally very costly. Besides, if you run out of fuel, the cells will not work.

Charging a boat battery without a charger

The most straightforward solution to charging a dead boat battery without a charger is jumpstarting it.

You only need a working battery and a set of jumper wires.

But having a jump starter will make everything easier.

Using a backup battery to jumpstart your boat

You can create a temporary power source by connecting the black jumper wire to the spare battery and the dead battery’s negative terminals.

Do the same for the red jumper wire by connecting it to the positive terminal of the dead and good batteries.

If your spare marine battery functions perfectly, it will start the dead battery instantly.

You may remove the jumper wires when the boat starts running because the alternator will start charging the battery.

Starting your boat directly using a jump starter

Jump-starters are battery-powered devices designed to jumpstart vehicles and boats.

They work similarly to jumper wires but do not need a second vehicle/boat to deliver power.

You only need to connect the battery to the jumpstarter by attaching the red wire to the dead battery’s positive terminal.

Similarly, connect the black cable to the dead battery’s negative terminal.

Immediately the boat starts, disconnect the wires.

Selecting the right charger for your marine battery

When it comes to selecting a boat battery charger, there are some factors you should consider to make a more informed buying choice (decision).

Battery charger types

The first thing to do while determining the exact type of battery charger for your boat is to understand the kind of battery you have. Do you need a gel, flooded, or AGM battery charger?

Also, you need to determine its parameters;

Battery capacity. It is usually indicated on the battery as “Ah.” Then, what Amp rating do I need for my charger?

To determine or understand the amp rating you require, you should perform some basic mathematics.

For example, assuming you have a group of 24V boat batteries with a rating of 85Ah. You need to multiply it by 10%, which gives you 8.5.

Therefore, you need an 8-Amp battery charger to recharge this boat battery.

Further, a higher Amp rating battery will have a much faster recharging process. And here is a breakdown of how fast your battery will recharge:

Amp ratingRecharge time (hours)
15 amps per bank3-5
10 amps per bank4-6
5 or 6 amps per bank10-12

Output voltage. The battery’s output voltage must match the voltage of your charger. Therefore, if your boat’s battery is 12 volts, you must pick a 12 volts charger.

This also applies to two 12-volt boat batteries connected in parallel because the voltage doesn’t double in this step. 

However, if your two 12-volts batteries are connected in a series setup, you will require a 24-volt charger because the output doubles to 24 volts.

Using a 24-volts charger to charge a 12-volt battery will destroy your boat battery and potentially cause a fire.

What if you need to recharge more than one battery?

Most boats use more than one battery to power everything from the starter to different quality-of-life accessories.

And the majority of boat battery chargers can charge anywhere between one to six batteries.

Therefore, we recommend a dual-output charger to recharge the service and starter batteries.

Battery bank

Battery bank

A battery charger meant for the marine environment

Choose a battery charger that is protected from the marine environment. For instance, it should be “tropicalized” to withstand salt spray.

Waterproof rating

The IP protection of your battery charger should be less or higher depending on the battery’s location.

For example, if a charger on a RIB is located in a trunk, where it is likely to be rinsed, you should choose a battery charger with an IP 65 water rating, which is an assurance that your charger is waterproof.

The charger’s ability to work in hot environments

Generally, the charging capacity of poor-quality chargers reduces with the temperature rise.

For instance, if your battery charger is put in an engine compartment, its performance could decrease rapidly when it reaches 40 degrees.

Resultantly, it increases the time required to recharge your marine battery.

So, select a battery charger that can effectively work beyond 50 or 60 degrees.


It is vital to note that some chargers have a ventilation system that is less or more noisy.

Therefore, if a charger is found in a berth on a sailboat, you should choose a silent charger, which helps preserve comfort on board.


Having a dead marine battery is a very unpleasant experience. However, following this guide lets you charge your battery quickly and easily and return to having fun on the water.

Still, we recommend getting a pair of jumper cables from Cloom in an emergency.