Charging Dual Batteries: Boat Edition

Charging a dual battery system in a boat is a straightforward process, but it's important to follow the correct steps to ensure the batteries are charged effectively and safely. The first step is to determine the type of battery setup you have. Most boats have a dual battery system with one starting battery and one deep cycle battery. Once you've identified your battery type, you can proceed with the appropriate charging method. It is recommended to use a 2-bank charger or a multi-bank charger to charge each battery separately. This is because, even though both batteries are 12-volts, they have different charging requirements and don't work well when charged together. Before charging, make sure to set the boat battery switch to the off position for both batteries. Additionally, it is crucial to use the correct voltage and charger that meets the specific requirements of your boat battery to avoid overcharging, which can damage the battery. When charging, always follow the directions on the battery charger, as extra steps may be required, especially if the battery is completely flat. It is also good practice to periodically check the water levels in your boat's battery and fill up when needed, unless you have sealed batteries which don't require this. By following these guidelines, you can effectively and safely charge your dual battery system in your boat.

Battery switches: a cheap and simple way to select one or both batteries

A dual battery system is a common feature on boats, especially those with two engines. It is a simple and cheap way to ensure you can always start your boat, even if one battery is depleted.

The system uses a battery selector switch with four positions: 1, 2, BOTH (sometimes 1+2) and OFF. This switch allows the user to select which battery is used to start the engine and receive a charge, or to combine the power of both batteries. The switch is usually labelled to indicate which position is which, and it is important to understand how it works to avoid accidentally draining both batteries.

The BOTH position is useful when batteries are weak and need to be combined to start the engine. However, it is not recommended to operate in this position for extended periods. For boats with twin engines, it is best to select Position 1 on one switch and Position 2 on the other, so both batteries are being used and charged. For boats with a single engine, alternate between Position 1 and 2 routinely.

The OFF position should only be used when the engine is not running, as passing through it when the engine is on can damage the engine's charging system. Even with the switch in the OFF position, the bilge pump automatic float switches will still operate.

It is important to note that a dual battery system requires careful installation and maintenance. The batteries must be secured to prevent movement and short circuits. The connections must be clean and low resistance to ensure efficient charging and starting.

Upgrades to a dual battery system are possible and can include adding a charge management device, such as an Automatic Charging Relay (ACR), to automate the charging process and protect sensitive electronics.

Twin alternators: a totally independent means of charging the start battery

A dual battery system is a great way to ensure you don't drain your boat's battery. It's also a good idea if you want to add electrical accessories to your boat, such as a powerful stereo subwoofer system, cabin lighting, or electric windlass.

One option for charging two battery banks is to install twin alternators. This setup provides a totally independent means of charging the start battery and a built-in redundancy in case a battery or alternator fails.

PBO editor David Pugh installed an extra 10A alternator on his 13hp Lombardini engine to completely separate the start and house batteries and their circuits. This setup also allows for the batteries to be combined if one battery fails using a switch. Additionally, if one alternator fails, the batteries can be combined, and the remaining alternator can be used to charge both batteries.

This setup is ideal if your engine is slightly overpowered for your boat, as it can make use of the excess power to run the additional alternator. It also means you can use a smaller, motorcycle-sized battery, reducing weight on board.

However, there are a few things to consider before opting for twin alternators. Your engine needs to be able to accommodate another alternator and be powerful enough to handle the extra load. Additionally, if you have two 'smart' alternators with external regulators, they may not always work well together. For example, during the bulk stage, both alternators will be charging the battery, but as the voltage increases, the one with the lowest threshold will switch out of bulk earlier than the other, causing the voltage to drop and locking the other one in bulk until it switches.

If you're considering twin alternators, it's essential to consult with a marine electrician or a professional with experience in boat electrical systems to ensure your engine can handle the additional load and that your specific setup will work as intended.

Split-charge diodes: a 'fit and forget' way to charge two battery banks

A dual battery system is a great way to ensure you don't run out of power on your boat. It can lend peace of mind and is an easy project to take on.

Split-charge diodes: a fit and forget way to charge two battery banks

Split-charge diodes have been used since the 1960s to charge two battery banks independently. They are a 'fit and forget' solution, allowing you to isolate and charge two battery banks without having to remember to switch between them. This is especially useful if you want to run multiple appliances and still be able to start the engine.

A diode is a one-way valve for electricity, allowing current to flow in only one direction. A split-charge diode system uses two diodes to split the charge from an alternator and deliver it to two separate batteries. This keeps the batteries totally isolated from each other.

However, diodes are not a perfect solution. They cause a voltage drop of up to 1V as current passes through them, meaning the batteries will not receive the alternator's full charge. This problem can be overcome by using a battery-sensed alternator, which will enable the batteries to charge fully.

Additionally, it is advisable to fit a switch so that the batteries can be combined in an emergency. Without this, the batteries will be completely separated, and you won't be able to start the engine using the house battery.

Another drawback is the energy wastage caused by diodes. The energy lost is converted into heat, which can be seen in the large cooling fins on diode systems.

Despite these drawbacks, split-charge diodes are a simple and effective way to manage a dual battery system, especially when compared to the traditional method of using a switch to select one or both batteries. This traditional method relies on the user remembering to switch between batteries and can result in both batteries being drained if the user forgets to switch.

VSRs: a simple solution that takes human error out of the equation

VSRs, or Voltage-Sensitive Relays, are a simple solution to charging a dual battery system in a boat. They are also known as ACRs or Automatic Charging Relays. VSRs are automatically controlled relay switches that allow two batteries to be combined when charging and isolated when not charging. This means that when a boat is running, both batteries will be charged, but when it is not, the batteries will be isolated, and current will be drained from only one of the batteries. This ensures that the start battery always remains fully charged and ready to start the engine.

VSRs are a modern alternative to the traditional battery switch, which requires manual switching between batteries and relies on the user remembering to make the switch at the right time. With a VSR, there is no risk of human error, as the system automatically senses which battery has more current and draws the charge to that battery. This also means that both batteries are maintained evenly and are ready in case of emergencies.

VSRs are especially useful when a boat has a small inboard or outboard engine with combined starter motor and alternator wires to the battery. In this case, a diode system will only allow current to flow one way, whereas a VSR will allow for both charging and starting with only one set of wiring.

VSRs are a simple, effective solution for dual battery systems in boats, ensuring that batteries are charged correctly and reducing the risk of being stranded due to a flat battery.

Solar battery chargers: a good option to keep your battery active

Solar battery chargers are an excellent option for boat owners who want to keep their batteries active and ensure operational readiness. They are a popular choice for those seeking a sustainable and self-sufficient solution for their vessels. By harnessing solar energy, these chargers offer a green and cost-effective alternative to traditional power sources, reducing reliance on shore power and fossil fuels.

Benefits of solar battery chargers

Solar battery chargers offer a multitude of advantages for boat owners:

• Eco-friendliness: Solar energy is clean and renewable, helping to reduce carbon emissions and contributing to environmental conservation efforts.
• Cost efficiency: While there is an initial setup cost, solar charging eliminates the ongoing expense of purchasing fuel or paying for shore power, resulting in long-term savings.
• Low maintenance: Solar chargers require minimal upkeep, making them a hassle-free option as they have no moving parts, reducing the chance of failure.
• Energy autonomy: Solar chargers provide energy independence, empowering boaters to explore further without the worry of locating recharging stations.
• Battery health: The consistent and gentle charging provided by solar chargers helps protect the battery from the wear and tear caused by fluctuating voltages.

Types of solar battery chargers

Solar battery chargers come in a variety of forms to cater to different needs:

• Portable solar chargers: These are compact and lightweight, making them ideal for small boats or quick charging on the go. They are perfect for charging small electronic devices and fishing gear batteries.
• Fixed solar panel chargers: These are larger, permanently mounted options that are designed to withstand marine environments. They are suitable for maintaining the charge of main and auxiliary batteries in larger recreational boats, yachts, and sailboats.
• Flexible solar chargers: These chargers are made with thin-film solar cells, allowing them to bend and adapt to curved surfaces. They are lightweight and can be mounted without significant structural support, making them ideal for boats with limited flat space.
• Solar trickle chargers: These chargers provide a small, continuous charge to prevent natural battery discharge during periods of inactivity or storage. They are suitable for all types of boats.
• Solar panel kits with controllers: These kits include solar panels, a charge controller, and sometimes additional accessories. The charge controller is essential for preventing overcharging.
• Marine-grade solar chargers: Designed specifically for the marine environment, these chargers are resistant to saltwater corrosion, high humidity, and are often waterproof. They are a necessity for boats regularly exposed to harsh marine conditions, including commercial vessels and sailboats on long voyages.

Choosing the right solar battery charger

When selecting a solar battery charger, it is crucial to consider several factors:

• Energy consumption: Calculate the daily power usage in watt-hours (Wh) by listing all electrical devices and their consumption.
• Battery capacity: Understand the total capacity of your battery in amp-hours (Ah) to ensure the charger can fully charge it during sunlight hours.
• Sunlight exposure: Take into account the average sunlight hours your boat will receive, which can vary by location and season.
• Charger efficiency: Consider the efficiency of the solar charger, as not all captured sunlight is converted into usable electricity.

A basic formula to estimate the required wattage of the solar panel is:

> Required Wattage = Daily Energy Consumption (Wh) ÷ Sunlight Hours ÷ Charger Efficiency

For example, if your boat consumes 600Wh per day, receives 5 hours of sunlight, and you choose a charger with 75% efficiency, you would need:

> Required Wattage = 600 ÷ 5 ÷ 0.75 = 160 watts

It is recommended to include a margin for cloudy days and future power requirements when making this calculation.

Frequently asked questions