Understanding Boat Battery Chargers: A Guide To Marine Power

Boat battery chargers are essential for maintaining the health and longevity of your boat's batteries. There are two main types of boat battery chargers: portable and onboard. Portable chargers are less expensive and can be used anywhere, but they need to be manually connected and disconnected from the batteries for each charge. On the other hand, onboard chargers are permanently installed and wired directly to the batteries, making them more convenient to use. Other types of chargers include multi-stage chargers, also known as smart chargers, which optimise the charging process by providing the appropriate voltage and amperage at each stage, and DC chargers, which connect to the boat's alternator and charge the batteries when the outboard motor is running. When selecting a boat battery charger, it is important to consider factors such as battery compatibility, charging speed, and the number of charging banks needed for the boat's batteries.

Onboard vs. portable chargers

Onboard and portable chargers each have their own advantages and disadvantages. This section will outline the pros and cons of each type of charger to help you decide which one is best for your boat.

Onboard Chargers

Onboard chargers are permanently installed on your boat, which means your entire charging system is already wired. All you need to do is plug the charger into a 120-volt outlet to begin charging your batteries. Onboard chargers are typically more technologically advanced and feature multi-stage charging capabilities, which can help prolong the life of your marine batteries. They are also very convenient as they can charge multiple batteries simultaneously, saving you time. Additionally, most onboard chargers will come with protection against overcharging, usually in the form of a float or maintenance mode. This means you can keep the charger plugged in without worrying about damaging your battery.

However, onboard chargers tend to be more expensive and add weight to your boat. They also need to be installed, which can be done by following the manufacturer's instructions or by hiring a professional. When installing an onboard charger, be sure to leave some space between the charger and the batteries while ensuring that the cables can still reach the battery terminals.

Portable Chargers

Portable chargers are generally cheaper than onboard chargers and can be easily moved from one battery to another. They are a good option if you need to charge your batteries away from your boat. However, they can be inconvenient to set up and switch between batteries in the confines of a boat's battery compartment.

Since portable chargers are not permanently installed, they do not add any weight to your boat. This is beneficial if you want to keep your boat as lightweight as possible. Additionally, with a portable charger, you don't have to worry about forgetting it at home since it's not permanently installed.

Charging multiple batteries

There are several ways to charge multiple batteries:

Battery Switches

The traditional way to switch between batteries is with a switch that allows you to select one or both batteries. This is a cheap and simple method, but it relies on the user remembering to switch it over to the correct battery.

Twin Alternators

Some boats have an additional alternator, which provides a totally independent means of charging the start battery and a backup in case of failure. This method requires the engine to be able to take the extra load of another alternator.

Split-Charge Diodes

Split-charge diodes have been around since the 1960s. They allow current to flow in only one direction, splitting the charge in two and delivering it to two separate batteries which are totally isolated from each other. This is a 'fit and forget' solution, but diodes create a volt drop of up to 1V, so batteries won't receive the alternator's full charge.

Battery Combiners

Battery combiners, also called Automatic Charging Relays (ACRs) or Voltage Sensitive Relays (VSRs), connect two battery banks in parallel when there is a charging voltage and keep them separate when there is no charging voltage. This is a good solution for boats with outboard or two small, similarly-sized battery banks.

Battery Isolators

Battery isolators allow one charging source to charge multiple battery banks without the banks being connected to each other. They are useful when batteries are of different sizes or chemistries. Modern FET battery isolators are more efficient than older diode isolators, which generate heat and waste energy.

Charging times

The time it takes to charge a boat battery depends on the type of battery and charger you have. To determine a battery's charging speed, you need to know two things: the amp-hour rating of your battery and the amperage of the battery charger. Once you have this information, you can calculate the charging time by dividing the battery's amp-hour rating by the amperage of your charger. For example, a 75Ah battery using a 7.5 amp charger will take roughly 10 hours to fully charge.

A charger with 15 amps will take about two hours to fully charge a battery at a 25% discharge value. Chargers with only 5 amps, on the other hand, will need a full six hours to charge the same battery.

Lithium marine batteries offer faster charging times compared to traditional deep-cycle batteries. They can handle higher charge and discharge rates and require a carefully controlled charging process, with built-in battery management systems that prevent overcharging and overheating.

Smart chargers, which are available in both portable and onboard varieties, are designed to optimise the charging process, ensuring that marine batteries are charged efficiently and safely. They employ a multistage charging approach, including bulk, absorption, and float stages. This method speeds up the charging process and safeguards the battery's longevity by preventing overcharging.

When selecting a boat battery charger, it is important to consider factors such as charger type (portable or onboard), battery compatibility, charger size, and the number of charging banks if your boat has multiple batteries.

Preventing overcharging

Understand Your Battery Type:

Firstly, it's important to know that boats usually have two types of batteries: starting batteries and deep cycle batteries. Starting batteries provide a powerful burst to ignite the engine, similar to a car battery, and they are not meant for deep discharging. Deep cycle batteries, on the other hand, provide steady power over an extended period to run electronic equipment like trolling motors, GPS, and radios. Only deep cycle batteries typically need to be recharged regularly.

Choose the Right Charger:

Selecting a suitable boat battery charger is essential. Consider using a "smart charger" or a charger with a float/maintenance mode. Smart chargers have a computer chip that monitors the battery voltage and automatically stops the charging process once the battery is full, preventing overcharging. Chargers with a float mode will switch off automatically once the battery reaches its ideal state of charge and turn back on when the battery drops below the optimal level.

Charging Time and Voltage:

Understanding how long to charge your battery is vital to prevent overcharging. You can calculate the charging time using the formula Ah/A = T, where Ah is the amp-hour rating of the battery, A is the charger output in amps, and T is the time. Always refer to the manufacturer's guidelines for charging times and voltage recommendations to avoid overcharging.

Regular Maintenance and Monitoring:

Keep your battery charged when not in use, especially during the off-season. Regularly inspect your battery for any signs of corrosion, leaks, or damage. Check the electrolyte level monthly to ensure the fluids are at the correct level. Additionally, consider investing in a battery monitor to keep track of your battery's status.

Battery-Specific Considerations:

If you have lithium-ion batteries, pay close attention to the charging profile. Default "lithium" profiles may not be optimal for your specific battery, and customisation may be required to prevent overcharging. Additionally, consider the impact of charging on other components, such as the alternator, wiring, and charging sources.

By following these guidelines and staying vigilant, you can effectively prevent overcharging your boat battery, ensuring its longevity and avoiding potential damage to your boat and its electrical systems.

Battery maintenance

Maintaining your boat batteries is essential for keeping them in good condition and ensuring they last a long time. Here are some detailed tips for battery maintenance:

Battery Types

Firstly, it's important to understand the different types of boat batteries and their specific purposes. There are three main types of marine batteries: starting batteries, deep-cycle batteries, and dual-purpose batteries. Starting batteries provide a high burst of power to start the engine and are not designed for deep discharging. Deep-cycle batteries, on the other hand, provide steady power over an extended period and can withstand deep discharging. Dual-purpose batteries combine the characteristics of both starting and deep-cycle batteries but are not as efficient as dedicated batteries.

Charger Types

There are two main types of boat battery chargers: portable and onboard. Portable chargers are convenient for charging batteries away from the boat, while onboard chargers are permanently installed and allow for simultaneous charging of multiple batteries. Onboard chargers are generally more technologically advanced and feature multi-stage charging capabilities, which can help prolong battery life.

Multi-Stage Charging

Multi-stage charging, often conducted by "smart chargers," is an important feature to look for in a boat battery charger. These chargers have microprocessors that regulate the charging process, ensuring that batteries receive the appropriate voltage and amperage at each stage, resulting in faster and more efficient charging. The three stages of multi-stage charging are the bulk stage, absorption stage, and float stage, each delivering specific voltages and amperages to optimise the charging process.

Charger Selection

When selecting a boat battery charger, it is crucial to choose one that is compatible with the chemistry and voltage of your boat batteries. The charger should also be fully waterproof, resistant to corrosion, and able to be mounted in damp or wet locations. Additionally, consider the number of charging banks you need, depending on the number of batteries on your boat.

Charging Tips

To maintain the health and longevity of your boat batteries, follow the manufacturer's recommendations for charging and maintenance. Regularly inspect your charger and battery connections for any signs of corrosion or damage, and clean them as necessary. If you're storing your boat for an extended period, remove the batteries and store them in a cool, dry location, using a battery maintainer or trickle charger to keep them charged. Periodically check your batteries' electrolyte levels and refill them with distilled water as needed.

Frequently asked questions