Hooking Up Dual Boat Batteries: Backup Power At Sea

Hooking up two batteries as a backup power source for your boat engine is a simple yet effective way to ensure you don't run out of power when sailing. The process is straightforward and can be done by following these steps:

Firstly, you need to select an appropriate battery switch for your boat. This switch will allow you to connect and control the power flow from two batteries. Make sure the switch is designed for two batteries and can handle the current when starting the engine.

Next, you need to connect the second battery to the boat. Choose a marine battery that meets the engine specifications and complies with safety standards. Wet-cell batteries should be secured in a battery box, while absorbed glass mat batteries don't require any packaging.

Now, you must choose a surface to mount the battery switch. The switch should be placed as close to the batteries as possible and be easily accessible. It can be flush-mounted for a cleaner look but ensure it's protected from water and corrosion.

The next step is to connect the positive and negative cables. All cables should be clearly labelled as marine cable and be colour-coded for easy identification. Connect the positive terminals of both batteries to the corresponding posts on the back of the switch, then connect the positive cable from the engine to the output post of the switch.

Finally, connect the negative cables. A crossover cable between the negative terminals of the two batteries is necessary for the dual-battery system to function correctly. Also, connect the negative cable lug from the engine and the negative accessory wire to one of the negative battery posts, ensuring all connections are tight.

Now you have a dual-battery setup, providing peace of mind and ensuring your boat engine has backup power when needed.

Battery switch: A 1-2-BOTH marine battery switch allows you to select a battery, combine them or disconnect both

A 1-2-BOTH marine battery switch is a common feature on boats with two batteries: a "start" battery and a "house" battery. It allows you to manually control which battery is charged by your engine's alternator. This type of switch is useful when you do not have an automatic charging relay installed.

• OFF: Both batteries are isolated, and nothing is running. This is useful when storing your boat to prevent battery drain.
• POS 1: The house battery is isolated, and everything (house loads, engine cranking, and engine charging) runs off the start battery.
• POS 2: The start battery is isolated, and everything runs off the house battery.
• BOTH: The two batteries are combined. The engine charges both batteries while running, and the house loads drain both batteries.

When using a 1-2-BOTH battery switch, it is important to remember to manually change the switch position depending on the charge/discharge state. Additionally, if both batteries are paralleled, a severely drained battery can pull the charge from the good one, leaving neither with enough power to crank the engine. Therefore, it is crucial to monitor the batteries' charge levels and adjust the switch accordingly.

Upgrading to an automatic charging relay (ACR) can provide a more automated and hands-off approach to managing your boat's battery system. An ACR will automatically combine or isolate your batteries, eliminating the need for constant manual switching.

Battery types: You'll need a starting battery with high current rush capacity and a deep cycle battery capable of deep discharge without harm

When setting up two boat batteries as a backup, it is important to understand the different types of batteries and their unique functions. In this case, you'll need a starting battery and a deep cycle battery.

A starting battery, also known as a cranking battery, is designed to deliver a high current rush or burst to start an engine. It supplies a lot of current for a short period of time and needs to be recharged almost immediately. Starting batteries are similar to automotive batteries and are suitable for boats with minimal DC loads where the engine is always running, such as wakeboard boats, runabouts, and personal watercraft.

On the other hand, a deep cycle battery is designed to be regularly deeply discharged, using most of its capacity. It can endure repeated deep discharging and recharging without damage. Deep cycle batteries are ideal for boats with larger DC power requirements, such as sailboats and fishing boats. They are also essential for powering onboard DC loads like lighting, trolling motors, inverters, and other systems.

When choosing a starting battery, look for one that matches the type and size of your original battery to ensure sufficient cranking power. For the deep cycle battery, consider a quality option like a lithium deep cycle battery, which provides consistent power throughout discharge and is lighter compared to lead-acid equivalents.

By combining these two types of batteries, you can have a reserve bank to ensure your boat's engine will start, and you can also run bigger DC loads for longer periods.

Battery installation: Wet-cell batteries should be secured in a battery box, while absorbed glass mat batteries don't need boxes

When installing a second battery on your boat, it is important to consider the type of battery you are using and follow the appropriate safety guidelines. Wet-cell batteries, also known as flooded batteries, are typically filled with corrosive acid or alkali and are considered dangerous goods. When installing a wet-cell battery, it is crucial to secure it in a battery box to prevent leakage and spillage. The battery box should be acid-proof and sturdy enough to anchor the battery securely, allowing no more than one inch of movement in any direction. This is essential for both safety and performance, as a loose battery can be a physical and fire hazard. Additionally, the battery box should have proper ventilation to release the explosive hydrogen gas generated during charging.

On the other hand, absorbed glass mat (AGM) batteries do not require a battery box. AGM batteries are designed with a special glass mat separator that wicks the electrolyte solution, allowing it to be stored in a "dry" or suspended state. This unique design makes AGM batteries non-spillable, even if the battery case is damaged or tipped over. However, it is important to note that AGM batteries have specific charging requirements and should be handled with care to ensure optimal performance and longevity.

When installing either type of battery, it is crucial to follow safety guidelines. Batteries contain a significant amount of stored energy, and improper handling can lead to short circuits and electrocution. Always disconnect the positive battery cables from the terminals before working on the electrical system. Additionally, it is important to wear safety gear, such as safety glasses, to protect yourself from battery acid.

Battery switch installation: Pick a surface with enough space for the selector switch. It should be as close to the batteries as possible and easily accessible

When installing a battery switch, it's important to choose the right location. The American Boat and Yacht Council (ABYC) standards dictate that the switch should be placed as close to the batteries as possible while also being easily accessible. This ensures that you can reach the switch when needed and makes the wiring process more straightforward.

Selector switches have a footprint of about 4 to 6 inches wide, so make sure you select a surface that has enough space to accommodate the switch. If you want to achieve a neater appearance above decks, you can opt for flush-mounting the switch. However, it's crucial to ensure that the switch remains protected from spray and other elements.

Before permanently installing the switch, it's advisable to wait until you've connected the positive cables to the posts at the back. This will make the process more efficient and organised.

When it comes to the appearance of the switch, they usually aren't very aesthetically pleasing. Therefore, it's a good idea to choose a location that is out of direct view, perhaps somewhere tucked away but still easily reachable.

Additionally, you can explore innovative solutions like a Battery Switch Box. This option allows you to mount the switch flush, keeping it out of sight, protected from the elements, and easily operable.

Positive and negative cables: Connect the positive terminals of both batteries to the corresponding posts on the back of the switch, then connect the positive cable from the engine to the output post of the switch

To connect two boat batteries as a backup, you will need a battery selector switch, also known as a dual battery switch. This will allow you to choose between your two batteries, combine them, or disconnect both.

Positive and Negative Cables:

Connect the positive terminals of both batteries to the corresponding posts on the back of the switch. Then, connect the positive cable from the engine to the output post of the switch. Also, connect the positive accessory wire to the output post.

It is important to ensure that the stripped and crimped end of the cable is inserted into the connector terminal and then physically soldered or warped by compressing (crimping) it firmly around the wire. You can use crimping pliers or other specialised crimping tools for this. After crimping, seal the cable-lug connections with heat-shrink tubing. This will provide a protective seal to your battery connections and protect them from water and abrasion.

Finally, make sure all terminal nuts are tight, and consider replacing wing nuts with locking hex nuts for a more secure connection.

Frequently asked questions