Wiring Dual Batteries: A Step-By-Step Guide For Boats

Setting up dual batteries on a boat is a straightforward process that offers peace of mind and ensures you're not left stranded on the water. It involves adding a second battery to your boat, which can be a great backup solution if your primary battery fails or drains. This setup typically includes a battery selector switch, allowing you to choose between batteries, combine their power, or disconnect them. Here's a simple introduction to guide you through the process of hooking up dual batteries on your boat.

Choosing the right batteries

When selecting batteries, you need to consider both the starting load and the running load of your boat. The starting load refers to the power required to start the engine, and the running load is the power needed to run electronics like GPS, trim tab motors, and lighting. Most boats have at least two types of batteries: starting batteries and house batteries.

Starting batteries deliver a large amount of current (75-400 Amperes) to start the engine but can only do so for a short duration (less than 15 seconds). They are similar to automotive batteries and are available in flooded and sealed types. If you are installing a reserve starting battery, you can duplicate the type and size of your original battery.

House batteries, also known as deep-cycle batteries, are used to power electronics and lights. They can withstand repeated deep discharging and recharging without damage and are suitable for boats with larger DC power requirements. Lithium deep-cycle batteries are a great option as they are lighter and provide consistent power. If you are installing a "house" bank, investing in a quality deep-cycle battery is recommended.

If you have limited space on your boat, dual-purpose batteries can be a good option. They can power both the starting and house loads but are generally more expensive and have lower power capacity than dedicated deep-cycle batteries.

When choosing batteries, it is important to consider the cold-cranking amps (CCA) specification, which indicates how many amps a battery can deliver in 30 seconds at 0°F without dropping below the voltage specifications. Select a battery that meets or exceeds the CCA requirement of your boat engine.

Additionally, pay attention to battery safety standards from organizations like the American Boat and Yacht Council (ABYC). Use low-resistance, tinned copper wire, and ensure proper ventilation as batteries can release explosive hydrogen gas during charging.

Plan your layout

Planning the layout of your dual battery system is a crucial step in ensuring a safe and functional setup. Here are some detailed instructions to guide you through the process:

Firstly, decide on a suitable location for your batteries. Consider a place that is dry, well-ventilated, and easily accessible for maintenance and inspection. Keep in mind that batteries are temperature-sensitive, so choose an area with comfortable ambient temperatures. Avoid placing them in a hot engine room, as high temperatures can significantly reduce their lifespan. Additionally, ensure that the battery cables have short runs to the engine's starter or alternator to prevent voltage loss.

Battery Boxes:

Use battery boxes to secure your batteries and prevent them from moving more than one inch in any direction. Pre-manufactured, acid-proof battery boxes are available in various sizes and are recommended for safety. If you opt for a custom-built box, ensure it is constructed from marine-grade plywood, covered with fiberglass cloth and epoxy for durability and protection.

Battery Placement and Connection:

Decide how the batteries will connect to the boat's electrical system. Most boats with dual-battery systems use one starting battery and one deep-cycle battery. The starting battery is similar to an automotive battery and provides a large amount of current for a short period to start the engine. The deep-cycle battery, on the other hand, powers electronics and can withstand repeated deep discharging and recharging without damage.

Safety Considerations:

When planning the layout, always prioritize safety. Ensure that the battery area is well-ventilated, as batteries, especially flooded-type batteries, release explosive hydrogen gas during charging. Ventilation is crucial to prevent gas buildup and reduce temperature. Additionally, keep safety equipment nearby, such as safety glasses, a water hose, and a fire extinguisher, to handle any potential hazards during installation and maintenance.

Compliance with Standards:

Ensure that your battery installation complies with Coast Guard and American Boat and Yacht Council (ABYC) standards. ABYC publishes safety guidelines for circuit protection, wire sizes, and other electrical system-related issues. Refer to these standards to guide your layout and installation process, ensuring a safe and reliable dual battery system for your boat.

Gather materials

To hook up dual batteries on a boat, you will need to gather the following materials:

• Batteries: You will need two marine batteries that meet the engine specifications of your boat. Starting batteries are similar to automotive batteries and can supply a lot of current for a short period, while deep-cycle batteries can endure repeated deep discharging and recharging. Lithium-ion batteries are also an option, as they are lighter and provide consistent power.
• Battery Boxes: If you are using wet-cell batteries, you will need secure, acid-proof battery boxes to store them. Absorbed Glass Mat (AGM) batteries do not require boxes.
• Battery Switch: A dual battery switch, also known as a marine battery switch or marine dual battery selector switch, allows you to choose between batteries, combine them, or disconnect both. It should be designed for two batteries and handle the amperage when starting the engine.
• Cables: You will need marine-grade cables, typically size 2/0, in red and black. These cables should be low-resistance, tinned copper wire to withstand the marine environment.
• Cable Lugs: Marine-grade cable lugs that are either crimped or soldered are necessary to connect the cables to the batteries and switch.
• Heat-Shrink Tubing: This is used to seal and insulate the cable-lug connections, protecting them from moisture and corrosion.
• Terminal Caps: Non-conductive caps for each positive battery terminal to prevent short circuits.
• Tools: A power drill with drill bits, stainless-steel mounting screws, and a heavy-duty crimper or soldering gun and solder are needed for installation.
• Cable Management: Cable clamps and tie-wraps help to organise and secure the cables.

Once you have gathered all the necessary materials, you can begin the installation process by planning the layout, following safety precautions, mounting the batteries, wiring the system, and finally, testing your setup.

Follow safety precautions

Batteries contain a large amount of stored energy and are potentially dangerous. Therefore, it is important to follow safety precautions when hooking up dual batteries on a boat.

Firstly, always ensure that the battery terminals are detached from the cables. Disconnecting the wires from the terminals can protect you from short circuits or electrocution. Only use the prescribed size of wires and adhere to the safety precautions outlined in the manual for optimal protection.

Secondly, check the metal-to-metal connections. These connections should be clean, with the right-sized terminals that are properly crimped and waterproofed. Secure mechanical fasteners to ensure low resistance connections. For outboard engine applications, replace wingnuts on the tops of the battery with nylock nuts or hex nuts and lock washers that are tightened to at least 10-foot-pounds of torque. For inboard engine applications, use clamp-on battery terminals connected to the correct gauge of battery cable.

Thirdly, always wear safety goggles and gloves when working with batteries. Keep a water hose or another available method to flush battery acid out of your eyes and off your face. Be cautious when using unshielded droplights or any open sources of heat or flame around the batteries. Be extremely careful with uninsulated tools that could cause a short circuit if they come into contact with the battery terminals.

Finally, ensure that the batteries are properly secured in a safe location. The location should be comfortable for a person, as batteries are temperature-sensitive and high temperatures can shorten their life. Batteries, especially the flooded type, give off explosive hydrogen gas when charging, so they must be vented from the top directly outside the boat. If there is insufficient natural ventilation, add a sparkproof exhaust fan to eliminate gas buildup and lower the temperature.

Wire the system

To wire the system, you will need to plan the layout of your batteries and how they will connect to the boat's electrical system. Gather the necessary materials, including batteries, cables, a switch, and installation tools. Always disconnect the power and wear protective gear before starting any work.

Firstly, securely install the batteries in a dry and ventilated location on the boat, ensuring they are strapped down and cannot move more than an inch in any direction. Next, connect the positive terminals of the 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, along with the positive accessory wire.

It is important to use low-resistance, tinned copper wire and tin-plated copper connectors for all connections to ensure the least electrical resistance and the best corrosion protection. Use marine-grade cables and heat-shrink tubing to seal cable-lug connections.

After connecting the positive cables, move on to the negative cables. Install a crossover cable between the negative terminals of the two batteries to ensure the dual-battery system operates correctly. Also, connect the negative cable lug from the engine/ground and the negative accessory wire to either of the negative battery posts, ensuring all terminal nuts are tight.

Finally, test your setup to ensure everything is functioning correctly. Refer to resources like BoatUS for visual aids and more detailed instructions if needed. If you are not confident in your ability to install the system safely, consult a professional.

Frequently asked questions