How to charge a Marine Battery

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Charging Your Marine Battery

An important part of preparing for the boating season is the power supply, so maintaining and charging your marine battery or batteries is a crucial aspect of ensuring a seamless and worry-free boating experience.

The first step is to maintain charging even during the summer months. This will keep your boat’s battery in top condition, and you can be sure that the engine will start when you go out on your next trip.

The margin between a fully charged and a discharged battery is smaller than you think. A small drop in charge can jeopardise the health of the battery, as sulphate crystals can build up and degrade the battery, reducing its capacity and even requiring to be replaced prematurely. Battery failures can also damage or compromise the boat’s other electronics, resulting in unnecessary and high costs.

It really pays to get into a routine of regular maintenance charging, even during the summer months. Regular maintenance charging can extend the life of your battery by up to three times.

What Are The Types of Marine Battery Chargers?

Onboard marine battery chargers are permanently installed on the boat and are connected directly to the boat’s electrical system, making it easy and safe to recharge your marine batteries without removing them. These chargers offer the advantage of automatic charging, as they continuously monitor the batteries condition and adjust the charging rate accordingly. Onboard chargers are recommended if you mainly use your boat close to shore power sources or marinas, as this removes the need to carry portable battery chargers every time.

Shore power charging is used when your boat is docked in a marina or connected to an external power source, and shore power charging is the most common and convenient method of charging your marine batteries. Onshore charging relies on a direct connection to the electrical grid at your marina.

Offshore charging on the other hand does not use any electrical power from the grid and relies entirely on the use of generators or alternative power generation methods to charge the boat’s batteries. In order to supply sustainable power and remain self-sufficient while offshore, integrating solar power systems and wind systems is becoming increasingly more popular. These methods, as well as using generators are essential for long distance trips and extended voyages, meaning the boat can produce the required power needed to remain self-sufficient even when away from the mainland.

Alternator charging occurs when the boat’s engine is running and the alternator can also feed charge to the batteries. Usually the charge from the alternator is primarily for charging the boat’s starting battery or batteries, although depending on electrical wiring it may be possible to have the domestic batteries receive some charge as well. Although bear in mind the priority for charging from the alternator should be the starting batteries, so as not to find yourself stranded at sea with low charge starting batteries due to the domestic batteries taking most of the charge. Additionally, alternator are mainly designed to supply a low current to starting batteries to maintain their level of charge, as opposed to charging a low or flat battery which will be too much of a task for the alternator.

Portable marine battery chargers are compact and versatile, designed to be carried on board or used on land to charge your marine batteries when docked. You will find different manufacturers produce a range of portable marine battery chargers, intelligent chargers, automatic and manual chargers, and depending on the chargers setting, these can be used on various types of marine batteries.

Portable chargers offer mobility and flexibility for your boating experience which can be crucial if you are away from marinas, using different boat’s or perhaps whilst renting a boat. A portable charger offers convenience of carrying one single charger which can be used in diverse situations for your charging requirements and can be far more cost effective compared to installing onboard systems.

Compatible Chargers

When choosing your battery charger, it must be compatible with your batteries, whether you decide to go with portable, onboard or both. If you have a single battery on your boat then it is relatively straightforward identifying the voltage and ampere hour of your battery to determine which type of charger is required. If you have a single 12 volt battery then you should make sure your charger is a 12 volt charger. Where the ampere hour is concerned, a rule of thumb is 10% of the batteries capacity and this would be a suitable ampere hour of your charger. As an example, if you have a 100Ah battery then 10% of this would be 10amps, this would mean the ideal charger would be a 10 amp charger.

Some boat’s may have two batteries onboard, this may be two starter batteries, or could be one starter battery and one for the domestic electric’s and appliances. When another battery is added to your marine system the approach to charging becomes a little more complicated. Compatibility of chargers will depend on the way the batteries are connected in your marine electrical system. The primary configurations are: Series and Parallel and the charging requirements are very different. You can learn more about series and parallel connection here.

If you connect batteries in series, then the voltages of the batteries are added together. An example of this is: if you take two 12 volt batteries and connect them in series, then the total voltage of the batteries would be 24 volts. If you have your batteries connected in this way then you would need a 24-volt battery charger for your system.

If you connect your batteries in parallel, then the voltage of the batteries remains the same, however the amper hour of the batteries is added together. An example of this is: if you take two 12 volt batteries and the amp hour is 100Ah for each battery, and you connect them in parallel you would still have a 12 volt system but double the capacity. This would mean you have a 200Ah system and as such you would require a 12 volt charger that can charge a 200Ah battery bank, and as per our previous 10% rule of thumb you would need a 20Ah battery charger.

Smart Chargers and Multistage Charging

Modern Smart chargers are an excellent choice for marine use due to their advanced algorithms and technology and the critical importance of maintaining healthy marine batteries. You will find both onboard and portable chargers can come with smart charging capabilities but it is always advisable to check prior to purchasing your chargers on their specifications. Smart chargers are designed to optimise the charging process and ensure that your marine batteries are charged safely and efficiently.

Wet Deep Cycle, AGM and Lithium

Charging wet deep cycle or AGM deep cycle batteries are similar, you may just find the charge rate of the AGM batteries is slightly higher, but typical lead-acid batteries require a specific charging regime. They need a multistage charging process which includes bulk charging, absorption and float charging which is all handled by the smart charger to ensure optimal charging this gives the battery the most performance and longevity available. Over charging and under charging can significantly reduce the lifespan and efficiency of your batteries.

Lithium batteries however use completely different technology and have a different charging profile. Lithium batteries require a careful charging process and can handle a higher charge rate and offer faster charging times compared to traditional lead-acid batteries. Lithium batteries have a built in BMS (Battery Management System) which prevents overcharging and overheating.

Battery Charging

How long you charge a marine battery for will depend on its level of discharge and the type of technology of the battery. If a marine battery has been subject to extremely deep discharge, you may find that it’s ability to recover has been significantly impaired, if this occurs it will never regain its former performance.

Series Vs. Parallel Connections Explained

While researching about batteries you may have seen the term series and parallel mentioned. We are frequently asked the questions like “what’s the difference between series and parallel?” and “can my batteries be connected in series?”. It can be confusing if you’re new to batteries in general, but hopefully, we can help simplify things.

Let’s start at the beginning – your battery bank. The battery bank is the result of connecting two or more batteries together for a single application (i.e. a boat). What does joining more than one battery together accomplish? By connecting the batteries, you either increase the voltage or amp-hour capacity and sometimes both, ultimately allowing for more power and/or energy.

The first thing you need to know is there are two primary ways to successfully connect two or more batteries: The first is called a series connection and the second is called a parallel connection.

Series Connections

Series connections involve connecting 2 or more batteries together to increase the voltage of the battery system but keeps the same amp-hour rating. Keep in mind in series connections each battery needs to have the same voltage and capacity rating, or you can end up damaging the battery. To connect batteries in series, you connect the positive terminal of one battery to the negative of another until the desired voltage is achieved. When charging batteries in series, you need to utilise a charger that matches the system voltage. We recommend you charge each battery individually, with a multi-bank charger, to avoid an imbalance between batteries.

In the image below, there are two 12 Volt batteries connected in series which turns this battery bank into a 24 Volt system. You can also see that the bank still has a total capacity rating of 100 Ah.

Parallel Connections

Parallel connections involve connecting 2 or more batteries together to increase the amp-hour capacity of the battery bank, but your voltage stays the same. To connect batteries in parallel, the positive terminals are connected together via a cable and the negative terminals are connected together with another cable until you reach your desired capacity.

A parallel connection is not meant to allow your batteries to power anything above its standard voltage output, but rather increase the duration for which it could power equipment. It’s important to note that when charging batteries that are connected in parallel, the increased amp-hour capacity may require a longer charge time.

In the example below, we have two 12 Volt batteries, but you see the amp-hours increase to 200 Ah.

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