How to Charge a Marine Battery : An Expert’s Guide

How to Charge a Marine Battery

Imagine a guy named Joe. Now Joe had a bad day out in the waters. It was hot and sticky.

The fish weren’t biting and he got sunburn. He just wanted to head back home to a nice hot meal and a cold beer.

And then the boat battery died.

I think we all can feel Joe’s pain. Now if Joe had paid more attention to his marine battery, this probably wouldn’t happen.

Batteries are not a huge mystery. We just need to understand this essential piece of kit to prevent getting stuck out in the open with no power.

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Types of Marine Batteries

Firstly, let’s get to know the types of marine batteries.

We have the starting battery, also known as a cranking battery. It is constructed with internal plates placed close together, giving more surface area for a one time high discharge.

Starting batteries, as it says on the tin, give that boost of energy to ‘crank’ or ‘start’ the engine.

Think of the starting battery as the hare in Aesop’s The Tortoise and the Hare. The hare can shoot off the starting block like a bullet.

Next we have the deep cycle marine battery. If the starting battery is the hare, the deep cycle marine battery is the slow and dependable tortoise.With its thick active plates, it gives consistent power through a slow, steady discharge; perfect for the trolling motor, plumbing system and other instruments on board such as navigational drives.

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Then there are dual batteries, capable of handling both needs. Most leisure boats need both starting and deep cycle and the temptation is to go immediately for the dual type; rather than two separate batteries.However, in the long run, the dual battery is not as efficient nor as durable as a dedicated deep cycle battery.


Chemical Make-up

Secondly, what are your batteries made of?

All the three types of marine batteries above work on the basis of a chemical reaction producing a discharge.

There are three distinct chemical types; the most common being the flooded or ‘wet cell’ batteries.

These are relatively inexpensive and with proper maintenance, can last years.

The downside is that they need to be stored upright and could not withstand vibrations; both difficult when you are in a boat. These batteries are also not sealed and could be a hazard when charging.

In comparison, gel batteries are sealed, hence safer. The chemical inside is a vicious gel which will not leak if the casing is compromised. However, gel batteries have a tendency to be overcharged and will not last long.

The newest kid on the block is the superior AGM (absorbent glass mat technology) batteries.  Shock and vibration proof, durable and easy to recharge – they ticked all the right boxes. Before you rush out to buy one, you will need to spend serious money on a smart charger for AGM batteries. That is something to consider.

How to Charge a Marine Battery

It is recommended that you charge your battery even if it is new. New batteries need to be optimized and several cycles of charging to increase their capacity.

To avoid situations like Joe’s, please charge your battery after every fishing expedition. Ideally, your battery should be on a 50-80% state of charge at all times. Batteries tend to discharge even when inactive.

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So if our friend Joe here, had not maintained his battery during the winter season, it is likely why he is in this predicament. A dead battery can be charged back to life but will not be dependable.

Types of Marine Battery Chargers

Types of Marine Battery ChargersTypes of Marine Battery Chargers

The type of charger you need will depend on the chemical make-up of your battery.  Do not be tempted to get a cheap charger. It pays to have a good one.Low cost chargers where you have to ‘guess’ when the battery is ready, will either undercharge or overcharge your battery.  This will be detrimental to your battery’s life in the long run.

Another option to think about is whether you want a portable or onboard charger.

Of course a portable one is easier to move and probably be much cheaper; but it can be inconvenient to hook-up. Being portable, it could be easily stolen. Onboard chargers are definitely more expensive to install but it wins in terms of convenience. You will just have to plug and go.

Single Stage Charger

For flooded cells batteries, you can use a standard lead acid charger.

Flooded cell batteries should only be charged when the hydrometer indicates a specific gravity of below 1.225 and load test of below 9.6 volts. Charging is a quick and easy process with this out of the box.

Smart or Multi-stage Charger

If you have opted for the gel or AGM battery, you will need a smart or microprocessor charger. As the name implies, the charge will be in stages – the bulk, absorption and float charge.

The multi-stage approach ensures charging is optimized without damaging the battery’s internal components.

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Last but not Least, Safety Precautions

Marine batteries contain components that could be hazardous. Here are a few tips on how to safely charge your batteries:


  • Always read the manufacturer’s manual before starting.
  • Always use a well ventilated area for charging.
  • Protective clothing, especially goggles and gloves should be worn.
  • Make sure the charger is unplugged before connecting or disconnecting with the battery.
  • Frozen batteries need to be thawed out before charging.
  • If battery feels hot, it might be overheating. Stop charging, unplug and disconnect the charger and check with the manufacturer.
  • Do not jiggle the leads while charging. This might cause a spark which might lead to a fire.
  • Always clean and keep the batteries well-maintained between charges.

How to Charge a Marine Battery FAQ

Q. Can I charge marine battery by using my car charger?You can do so. But ensure the charger settings are correct.

So, there we have it. If our friend Joe pays attention to his battery and keep it well charged, he will not be having such a bad day out at sea.

He would be home already having a cold brew.However, a wise man once said, “A bad day out fishing is better than a good day anywhere else”

so maybe Joe is not that upset after all. Ta-da!


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