How to Charge a Marine Battery

Written by John Ho on November 14, 2020
How to Charge a Marine Battery

Harnessing solar energy to meet energy requirements for homes, businesses, and industries is one hurdle to cross in a quest to achieve green energy globally. Since the sun is only out half the day, there is a need for batteries to store energy for future use. The strength of a solar energy system depends majorly on two factors: the speed of charge and the Reserve Capacity (RC, usually in minutes) of the batteries. The RC tells a user the number of minutes it takes it to go from full charge to total discharge, for a constant load usually 25 amps. So, suppose you have a battery that has an RC rating of 180 minutes. What this means is that it would take about three hours to deliver 25 amps to your home before the battery is empty.

Batteries come in different types and meet different power consumption needs. You can also have some batteries come in different sizes: some very big and others relatively small. For this article, we would focus on the marine battery.

Marine batteries are a top choice when you seek a continuous supply of current as opposed to a vast discharge at one time. Marine batteries are built to withstand repeated vibrations and effects of waves. Hence they are very suitable for use on boats and marine equipment. Exposure to water waves and vibrations increases the chances of corrosion, material shedding, electrolyte leaks and eventually battery failure. To prevent this, they are made with heavier plates and have a more rigid construction. They are more powerful and usually more expensive than automobile batteries.

Marine batteries can be classified based on two main factors. We either group them based on the nature of electrolyte used or based on their functionality.

Types of Marine Batteries Based on Battery Technology

When we group marine batteries based on battery technology, or the type of electrolyte used, we have three types:

  1. Flooded Battery (or Wet Battery)
  2. Gel Battery
  3. Absorbed Glass Mat (or AGM) Battery
  4. Lithium-ion Battery

Before we look at them, let's know what an electrolyte is. An electrolyte is simply a substance that produces electricity when dissolved in water. We would understand this and its function as we progress.

Flooded Battery

Flooded batteries are batteries that function by using a liquid electrolyte. The liquid is usually sulphuric acid. They are the most common types of marine batteries and the cheapest too.

One advantage of using a flooded battery is that they are not damaged by overcharge, as the electrolyte solution produces gas when it is overcharging. As long as there is an escape for this gas, the battery will not get damaged due to the overcharge.

However, flooded batteries must be kept upright to prevent the electrolyte solution from spilling out. Also, there is the issue of evaporation of the solution and corrosion.

Gel Battery

Gel batteries make use of gel as its electrolyte. This gel is usually made up of sulfuric acid mixed with fumed silica and is thick or gel-like (hence the name). It lasts longer in hot weather or places of low ventilation. Also, you do not need to place it upright, unlike the flooded batteries, and the battery does not leak. They are the best type of batteries for deep cycle applications.

AGM Battery

The AGM, or Absorbed Glass Mat battery, is the most advanced of this category. The electrolytes are contained in the glass mat, and so they cannot spill out. It is safer, and it is temperature tolerant (up to -20°C) and vibration resistant. AGM batteries are also super-efficient, with low discharge rate. They can stay in storage longer than other battery types. They also require little or no maintenance.

These features make them suitable for military use, for which they were initially designed.

However, it is essential to pay attention while charging AGM batteries. They are not good with overcharge, and they can also get damaged due to undercharging.

Lithium-ion Battery

This type of battery is referred to as a high-performance battery, and they are a more recent type of marine batteries. They have some impressive features: they are light and occupy small space, they also charge very fast (about five times faster than the other types)

Types of Marine Batteries Based on Functionality

Marine batteries are also classified based on their functions. We have three types of marine batteries in this category:

  1. Starting Battery
  2. Deep Cycle Marine Battery
  3. Dual Purpose Marine Battery

Starting Battery

Starting batteries are also known as cranking batteries. They are used to supply an initial surge of power or ‘current bursts’ over a short time frame which is required to start the engine. They are also quickly recharged by the alternator. This battery type is very similar to the automobile battery.

Deep Cycle Marine Battery

Deep cycle marine batteries, as opposed to the cranking batteries, are designed to deeply discharge current in smaller amounts at steady rates over a long time. They can be repeatedly discharged and recharged without causing damage or shortening their life. They are used to power numerous electronics and accessories, or high-demand device applications. The deeper a battery is discharged, the fewer charge-discharge cycles it can perform.

Dual-Purpose Marine Battery

The dual-purpose or hybrid marine batteries are widely used in many applications. They combine the functions of both the cranking and the deep cycle marine batteries. However, they do not perform as optimally as the deep cycle marine batteries.

How to Charge a Marine Battery

Charging a marine battery, or any other kind of battery is a simple and straight-forward process. You only need the right information on the steps and procedures to take in doing this. Also, you need to be aware of some safety precautions while charging the batteries, so you do not sustain an injury or damage the batteries.

Charging your marine battery improves its efficiency and ensures that you get the best out of it. For new batteries, you should make sure it gets a proper and full charge before you begin using. It helps to improve the battery capacity. It is recommended that you charge up to 80% before use. 

Steps to Take When Charging A Marine Battery

Here are some essential guidelines to follow when charging your marine battery.

Step 1: Know Your Battery Type

Your battery type is the most basic information that you, as an owner, should have at your fingertips. Your battery type influences a lot of the decisions you would make while using it, including the type of charger to buy, how to handle it while charging, etc.

We have highlighted the types of batteries based on their categories, as you have seen earlier in the article. Let’s say Thomas has an AGM Deep Cycle Battery and Hilda just bought a Flooded Starting Battery. From the battery type, we can get necessary information about both batteries. For starters, Hilda can leave her battery charging and be unbothered, but Thomas wouldn’t. (If you don’t know why you might have to scroll up and read it up again)

Step 2: Make Sure You Are Using The Appropriate Charger

It is also crucial that you ensure that only the right type of charger is used on your marine battery. Using the wrong charger could permanently damage the battery, so you need to use only the correct specification.

Marine chargers come in different sizes. We classify them as either:

  • Portable chargers
  • Onboard chargers.

Portable Chargers

Portable chargers are mobile and can be moved easily from one point to the other. They are relatively cheap but may pose a bit of difficulty when hooking them up with batteries.

Onboard Chargers

Onboard chargers are permanently installed on the boat or marine equipment. They are much easier to use, but quite expensive.

We also have battery chargers grouped based on the mode of charging into:

  • Single-stage Chargers
  • Multi-stage Chargers.

Single-Stage Chargers

A single-stage charger has a single cut-off setting and charges the battery up to the cut-off voltage. It offers easy and fast charging and is suitable for Flooded Cell Battery type.

Multi-Stage Chargers

The Multi-stage Chargers are also referred to as smart chargers. They charge the batteries to their optimal levels, protecting the internal components while doing so. Also, they are faster and give full charge, thereby improving battery life.

Step 3: Connect The Battery Properly To The Charger

One quick instruction: make sure you clean the battery surface and wipe off any dirt or stain around the terminals and other parts. Also, observe the battery for any abnormalities. Do not connect to the charger if you notice anything strange.

To charge the battery, you connect the charger clips to the battery terminals. Usually, there is always a red and black clip. The red goes to the positive terminal while the black goes to the negative one. It is a simple task, but you can check your manual if you are confused about how to connect the clip. Monitor the battery to ensure that it is charging.

Step 4: Disconnect the Battery From the Charger At full Charge

As we discussed earlier, some battery types can get damaged from overcharge. It is always advisable to disconnect the battery from the charger when it is fully charged, irrespective of the kind to preserve it and even conserve energy. Turn off the charger and remove the clips from the battery terminals.

Safety Precautions While Charging A Marine Battery

  • Always make sure you go through the user guide or manual before you start handling both the charger and the battery.
  • Make sure your charging spot is well aerated. It is not safe to charge in a hot or stuffy place.
  • Take safety precautions for yourself. Put on appropriate safety gears while operating: gloves, boot, eye goggles, etc.
  • Make sure the charger is switched off before you connect the clips to the terminals.
  • If you notice that the charger is getting too hot during charge, disconnect and contact a technical expert.
  • Do not attempt to open the batteries for any reason. Report abnormalities to an expert.


Most marine batteries get damaged because of faults that arise due to poor charging techniques by users. So many of these little mistakes cause a lot of damage to your batteries. So, you must know your battery type and its features. Also, you are to follow all the stated guidelines while handling your battery during charge so that you can get quality returns for your money.

Article written by John Ho
Our goal is to provide you with comprehensive and thorough solar-energy product guides, which will leave you confident that you have made the right purchase decision for your home.

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