Battery Equivalents and Replacements

How to Charge a Marine Battery

Electrical demands on modern boats are much higher than a decade or more ago. Nearly all fishing boats have electrical devices such as lights, radios, sonars, refrigerators, laptops, electric trolling motors, electric watermakers, etc.

Updated: November 10, 2022.

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How To Charge Marine Battery Properly?

Marine batteries are charged using battery chargers/sources, depending on the battery type and type of electric system on the boat.

Smaller boats have one battery system, consisting of one battery, which may be:

  • Starting: found on small boats where the battery is used only for starting the engine and powering perhaps navigation lights when the engine is turned off.
  • Dual-Purpose: the battery is used for both starting, but also for powering various electric and electronic devices.
  • Deep Cycle: the battery is used for starting but also for powering a broad range of electric and electronic devices. Although deep cycle batteries are generally not suitable for starting applications, they are large enough to easily provide starting currents for smaller diesel and especially gasoline engines.

Having only one battery simplifies the electrical system, but if that single battery is discharged too much, it can be discharged too low and become unable to crank the main engine.

Hence the two-batteries electric systems are more popular, even on smaller boats. In that case, the batteries are:

  • Starting: used only for starting the main engine.
  • Deep Cycle: used for powering every electric device, gadget, or appliance, regardless if the main engine is ON or OFF.

To prevent issues between different batteries, starting and deep cycle batteries are separated by DC-to-DC battery chargers/charge controllers, allowing the user to have, for example, lead-acid starting battery and lithium deep-cycle battery or even a battery pack.

renogy 40a dc dc onboard battery charger

One such DC-to-DC battery charger is the Renogy 12V 40A DC-to-DC On-Board Battery Charger, which also supports the use of a D+ ignition cable and LC current limit cable.

Also, it allows the user to set battery chemistry and required charging voltages, which is very important for proper and safe battery charging.

Battery Chemistry and Charging Voltages

The most common marine batteries are:

  • Lead-Acid Batteries: Flooded/Wet, AGM and Gel,
  • Lithium Batteries: Lithium Iron Phosphate (LiFePO4), rarely Lithium-ion.

All these batteries must be recharged using dedicated smart battery chargers with properly set charging modes and voltages.

Charging algorithms differ between lead-acid and lithium batteries, and charging them with the wrong charging algorithm can damage the battery, overheat the battery, the battery can catch fire, etc.

So, before connecting the battery charger to the battery and the onboard power source, be sure to read the manual of your battery (or batteries) and be sure to connect the proper battery charger and to it set properly.

If You have several marine batteries to charge, feel free to check our Onboard Marine Chargers: Best 3 Bank Battery Chargers article.

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Intro to Boat Electric Power Sources

There are several ways in which power sources can be classified, but basically, they are shore-based or onboard generators.

Onboard generators can be gasoline, diesel, or even LPG internal combustion engines or electric power generators based on renewable sources, like solar panels or wind turbines.

Although most boats can generate electric power from their inboard/outboard engines and use it immediately and/or charge battery bank(s), more and more boats carry auxiliary generator(s), or they increase the size of their solar panels.

Since carrying enough fuel to power main engines and electric generator(s) over a long fishing trip can be a problem for several reasons, many fishing boats are equipped with secondary power generators such as solar panels, wind turbines, towed turbines, etc. that make fishing or leisure boat electrically self-sufficient over a long period of time, regardless of the amount and/or condition of stored fossil fuel.

Note: one of the reasons to have a secondary power source(s) are emergencies.

For example, your fuel gets contaminated with water (yes, I know, there are fuel filter systems that separate even water from fuel, but even they sometimes can fail!), making your main engine(s) useless, but also every other piece of equipment that runs on that fuel - electric generator powered by a separate internal combustion engine is useless, just as any other energy/power-hungry equipment like trolling motors (can be used in emergencies to provide propulsion), watermakers/desalinators/water filters (do you know how much water during summer heat single person need?), etc.

Long story short, more or less standard boat electric power sources are given in the following list.

Mains Battery Chargers

Mains chargers allow the batteries to be charged while the boat is in the port. Also, when in use, these chargers power the entire boat's electric grid - lights, electronics, refrigerators, etc.

Since larger boats can operate in different areas and in various countries, it is important that these chargers accept various voltages (110V, 220V, 380V), phases (single-phase or three-phase electric power), various sockets, etc.

A good boat charger/controller is an electronic device with several options (mains power, solar panel, wind turbine, etc.) for charging the main battery (or battery pack), monitoring currents and voltages, and sending alarms (audio, visual, SMS, etc.) if certain thresholds are met.

Wind Turbines

six blade wind turbineWind turbines harness wind energy. Blades are connected to the shaft that is connected either directly or over the transmission box with an electric generator.

They start to produce electric energy even during low wind speeds, but no wind, no electricity. Also, their blades spin, and in strong winds, they can spin really fast - wind turbines must be carefully positioned on the boat in order to avoid possible damages or injuries.

Solar Panels

Solar panels convert sunlight into electricity. Their power output depends on many things like the season, weather, the angle of the solar panels, their type, etc.

Unlike wind turbines, they have no moving parts and can be positioned more closely to the passengers and onboard equipment. They operate during the day, but during the night, they produce no electricity.

Very often, solar panels are combined with wind turbine(s).

Towed Generators

Towed generators, also known as hydro generators, are towed behind the fishing or leisure boat under sails. If water is deep enough, it is good practice to submerge the turbine generator deeper - up to 15-20 m, for better efficiency.

If you are on an electric boat or you are trolling using an electric trolling motor, it is useless to use a towed generator since it will cause more drag (and hence increase electric consumption by electric drive) than it will produce electricity.

Also, if you are anchored in strong sea currents (in channels, for example), you can use a "towed" generator to produce electricity while you are anchored.

Some systems have the possibility to use a rotation of the main drive propeller to power an electric generator while the ship is sailing.

These systems are not very energy efficient since the propeller must be optimized for boat propulsion, otherwise, such systems can lead to increased fuel consumption.

On the other hand, these systems are very handy and can charge batteries automatically - turn off the main engine and gearbox to neutral (regarding the main engine) and sails up, and you are producing enough energy to power lights and electronics and charge battery with few amps :)

Wave Generators

Also, there is a special type of generator I don't see very often - wave generators. They are positioned in the wake of the sailing ship or behind the stern of an anchored ship. They harness wave energy, but some V-shape models harness both wave and sea currents energy. They are still in development, so more on this some other time :)

Internal Combustion Engines

mercury 4 stroke outboardPractically every internal combustion engine has some sort of alternator/generator. They produce energy when the main engine(s) are running - to produce electricity, the main engine(s) must run at least on idle, which is not always the best solution.

Some boats have the main engine(s) coupled with generator(s) only - they are used to produce electricity (and maybe warm water), which is used to recharge batteries and drive separate electric motor(s) that are connected with propeller(s).

This way, internal combustion engines (diesel, petrol/gas) always operate at optimum conditions leading to reduced fuel consumption.

This system was even used on aircraft carriers, battleships, submarines, etc., and is still used on modern diesel-electric submarines, often combined with tanks of compressed or liquid oxygen and/or fuel cells - this system is found on modern German subs, Israeli subs, et. ... OK, if Germans are using them on subs, then it must be good ... :)

Portable Power Generators

Fixed or portable diesel or gas generators are commonly found on medium or larger fishing and leisure boats when electricity is in demand.

They have low fuel consumption and relatively low noise levels. Also, portable generators can be easily repositioned, for example, on-shore when required, or transported for maintenance, etc.

Larger models with remote control and electric starters are preferred for onboard use on larger boats. The output power of fixed and portable generators ranges from 1kW to 10kW, even more. So does their price.

Manual Power Generators

manual power generatorManual power generators provide power to the most basic equipment in need - radios and radio beacons, cell and smartphones, navigation lights, and similar.

They provide tens of watts of power, and they are not suitable for charging the main boat battery - while providing 2-3 Amps, they would require 10 hours to charge the 12V battery with 20-30Ah.

However, if you are unable to start the main engine manually due to a weak battery, recharging it with just 5-10Ah is sometimes more than enough to bring the battery to life just enough to start the main engine - if that is the case, after starting the main engine, go to the nearest port and don't turn off your main engine until you are safe in the harbor (after which, you will find some good electrician to check your electric system).

Fuel Cells

Fuel cells are new "toys for big boys" :) They use fuel to produce electricity with efficiency reaching 70-80% easily - for comparison, the best diesel engines rarely can achieve 45% efficiency, with 35-40% being normal; petrol/gasoline engines are even less efficient.

However, fuel cells are still very expensive and if you run out of fuel, guess what - they don't work, no matter how expensive they are :)

Flow Batteries

Flow batteries are something totally new - personally, I don't think that we will see them on the boats soon.

IMHO - it is vital to have a good and reliable main engine with an alternator that can recharge your batteries while running. To be sure, add some solar panels and, if you have room on your boat, one to two wind turbines - of course, be sure to insulate starting from deep cycle batteries by using a good DC-to-DC battery charger/charge controller.


 Frequently Asked Questions (FAQ)

Here are the most common Frequently Asked Questions (FAQ) about marine batteries and how to charge them:

Can I charge a marine battery with a regular charger?

If your "regular" battery charger allows you to set the battery chemistry, which ensures proper charging algorithm and charging voltages, then yes, in that case, you can use your "regular" charger.

Do you need a special battery charger for a marine battery?

If the "special" battery charger means setting proper battery chemistry, then yes, you need a "special" battery charger.

Note: most modern battery chargers allow the user to set the battery chemistry.

What Amp should I charge my marine battery?

Generally, lead-acid batteries should be charged with 0.1-0.15C currents, while Lithium Iron Phosphate (LiFePO4) batteries can be charged with 0.25-0.5C currents.

Note: many lithium models support much faster charging, with the 0.25-0.5C still being recommended.

Can you recharge a marine battery?

If You have a proper battery charger and power source, then yes, you can recharge the marine battery.

What is the best way to charge a marine battery? How to charge a marine battery?

The best way to charge the marine batteries is through the use of properly dimensioned advanced battery chargers, which allow the user to set the desired battery chemistry.