Connecting Multiple Batteries In Your Boat: A Step-By-Step Guide

Connecting two batteries in a boat is a great way to ensure you always have enough power and is a simple project for those with even limited electrical knowledge. The dual battery setup is a popular choice for fishing boats, allowing one battery to act as the starter battery and the other to power tools and navigation systems.

There are two main ways to connect two batteries: in series or in parallel. Series wiring increases the voltage of the battery pack, while parallel wiring increases the capacity rating. Both methods require the use of jumper wires to connect the batteries' terminals, with specific connections depending on the wiring type.

When connecting two batteries, it is important to use batteries of the same age, size, type, and manufacturer. It is also crucial to ensure the batteries have the same voltage and capacity rating to avoid charging problems and shortened battery life.

For boat owners wanting to add a second battery, there are a few tools needed, including cables, cable cutters, an adjustable wrench, grease, and a swagging tool.

Choosing the right battery switch

Number of Batteries and Engines

The number of batteries and engines on your boat will determine the type of battery switch you need. Most boats fall into one of the following categories:

• Single Battery, Single Engine: For this setup, a simple ON-OFF battery switch is sufficient. A Blue Sea 6006 or Bluewater ON-OFF battery switch is commonly recommended.
• Two Batteries, Single Engine: You can use either a 1-2-BOTH switch (e.g., Blue Sea 6007 or Bluewater 1-2-BOTH) or an ON-OFF-COMBINE switch (e.g., Blue Sea 6011 or Bluewater ON-OFF-COMBINE).
• Three Batteries, Twin Engine: You will need two ON-OFF-COMBINE switches (e.g., Blue Sea 6011 or Bluewater ON-OFF-COMBINE) or a combination of a Blue Sea 6010 and a Blue Sea 6006 (or Bluewater ON-OFF) with ACRs.

Amperage and Starting Requirements

Ensure that the battery switch can handle the amperage requirements of your electrical system and engine cranking current. A rating of 250 amps continuous is typically sufficient for most outboards and gasoline inboards.

Features and Functionality

Consider whether you need additional features such as locks for security, field disconnect circuits to protect the alternator, or automatic charging capabilities. More complex switches may offer more flexibility and control over your power usage.

Size and Accessibility

Battery switches come in different sizes, so choose one that fits the available space on your boat. Also, ensure that the switch is easily accessible and complies with safety standards, such as those set by the American Boat and Yacht Council (ABYC).

Ease of Use and Installation

Look for a switch that is easy to use and has clear, concise labeling. Consider the installation process and ensure that you have the necessary tools and skills to install the switch correctly. Follow the manufacturer's instructions and safety precautions during installation.

Durability and Reliability

Choose a battery switch made with high-quality, marine-grade materials that can withstand the harsh marine environment. Look for switches with a proven track record of reliability to avoid failures during your boating trips.

Cost

Finally, consider your budget. More advanced switches with additional features will generally be more expensive than simpler options.

Remember to consult a professional or a marine electrician if you are unsure about any aspect of the battery switch selection or installation process.

Connecting the second battery

Before connecting the second battery, it is important to ensure that the battery switch is designed for two batteries and can handle the current while starting the engine. A rating of 250 amps continuous is sufficient for most outboards and gasoline inboards.

The second battery should meet the engine specifications and comply with Coast Guard and ABYC standards. Wet-cell batteries should be secured in a battery box, while absorbed glass mat batteries do not need boxes. Both types require tie-downs or brackets, and the positive terminals should be covered with non-conductive boots.

The positive terminals of the two batteries should be connected to the corresponding posts on the back of the switch. Then, the positive cable from the engine should be connected to the output post of the switch, along with the positive accessory wire.

For the negative cables, a crossover cable between the negative terminals of the two batteries is necessary for the dual-battery system to function correctly. The negative cable lug from the engine/ground and the negative accessory wire should be connected to either of the negative battery posts, ensuring that all terminal nuts are tight.

When connecting the second battery, it is important to follow safety precautions. Batteries contain potentially hazardous substances, so it is crucial to detach the wires from the terminals to avoid high-risk short circuits or electrocution. Only use wires specified in the handbook, and be sure to follow all safety warnings.

Additionally, ensure that the metal-to-metal contacts are clean and in good condition. The terminals should be the correct size and watertight, and metal fasteners should be secure to achieve low-resistance connections.

Now, you can test if the connection is successful by attempting to crank the engine and checking if the accessories are functioning. If everything works as expected, clearly label the batteries to keep track of which is battery one and which is battery two.

Selecting a surface for the battery switch

When choosing a surface for the battery switch, it's important to consider the size of the selector switch, which is typically four to six inches wide. Therefore, you should select a surface area that can accommodate the switch and has enough space.

According to the American Boat and Yacht Council (ABYC) standards, the switch should be as close to the batteries as possible and easily accessible. This means that the surface you choose should be near the batteries and in a location that is easy to reach.

If you prefer a cleaner look, you can opt for a flush-mount installation above the decks. However, it is crucial to ensure that the switch is protected from spray and other elements.

Before finalising the installation, make sure to connect the positive cables to the posts in the back of the switch.

Joining the positive and negative cables

Firstly, ensure that the cables you are using are clearly labelled and marked as "marine cable". This is important for distinguishing the positive and negative cables and ensuring they are connected to the correct terminals. The cable size or gauge is also crucial; for most outboards and gasoline inboards, a size 2/0 cable is appropriate.

Now, let's get started with the connections. Connect the positive terminals of both batteries (Battery 1 and Battery 2) to the corresponding posts on the back of the battery switch. Typically, the switch will have numbered posts (e.g., #1 and #2) or labelled posts (e.g., Batt. 1 and Batt. 2). Make sure you connect the positive terminal of Battery 1 to the correct post (e.g., #1 or Batt. 1) and do the same for Battery 2.

Next, connect the positive cable from the engine to the output post of the switch. This will allow the engine to receive power from the batteries. Additionally, connect the positive accessory wire to the output post as well. This wire will power any auxiliary systems or devices on the boat.

At this point, you should have three connections made: the positive terminals of both batteries connected to their respective posts on the switch, and the positive cable from the engine and the positive accessory wire connected to the output post.

Now, it's time to join the negative cables. Connect the negative terminals of Battery 1 and Battery 2 together using a crossover cable. This step is crucial for the proper operation of the dual-battery system. Also, connect the negative cable lug from the engine/ground to one of the negative battery posts. Ensure that all terminal nuts are tight and secure.

As a final step, consider replacing any wing nuts on the battery terminals with locking hex nuts or Nylock nuts. This will provide a more secure connection and reduce the risk of loose connections.

By following these steps, you will have successfully joined the positive and negative cables when connecting two batteries in your boat. Remember to take appropriate safety precautions, work in a well-ventilated area, and always disconnect the battery cables from the terminals before beginning any work.

Connecting the negative cables to the positive cables

First, ensure you have the correct cables. You will need a 2/0 marine-grade cable in red and black. The varying colours will help you identify the cables for the positive and negative connections. The cables should have “marine cable” stamped on the insulation.

Next, connect the negative terminals of the two batteries with a crossover cable. This is necessary for the dual-battery system to operate optimally. Make sure that the stripped and crimped end of the cable is inserted into the connector terminal, which is then physically soldered or warped by compressing (crimping) it firmly around the wire.

After that, connect the negative cable lug from the engine/ground and the negative accessory wire to one of the negative battery terminals. You can use a heavy-duty crimper or a soldering gun and solder to make this connection. Tighten all terminal nuts and screws securely.

As an additional safety measure, consider replacing the wing nuts on the battery terminals with locking hex nuts or Nylock nuts. This will provide a more secure connection and reduce the risk of short circuits.

Finally, 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.

By following these steps, you will have successfully connected the negative cables to the positive cables in your boat's dual battery system. Remember to wear safety gear, such as gloves and goggles, and have a water hose nearby in case of any acid spills.

Frequently asked questions