How to Connect Dual Batteries in a Boat

Dual batteries are very common in boats. They allow boaters to relax on the water and enjoy the many appliances on modern pleasure boats such as stereos, lights, refrigerators, and even TVs without running down the starting battery. If your boat didn't come equipped with a dual battery setup, you can do it yourself. Installing a dual (or more) battery system isn't difficult. With a few simple tools, one person can install dual batteries in a single afternoon.

Things You'll Need

• Dual battery switch
• Marine sealant
• Drill (for pilot holes)
• Screwdriver
• Marine batteries
• Strap or bungee cord

Instructions

1. Buy a dual battery switch. The face of the switch has four possible settings: A, B, both, and off (some are marked 1, 2, both, and off). This allows you to choose between using one or two batteries or keeping them both on or off. Any of the first three positions are fine while running the engine, but never turn the battery switch to off while the engine is running. This will damage your engine.

2. Choose the type of battery you prefer. There are two types of marine batteries, regular, or starter, batteries, and deep cell batteries. Deep cell batteries have a longer life between charges but are more expensive. Use the same types for both of your batteries. In addition, it is a good idea to replace an original battery when you buy a second one, even if they are the same type. An older battery will cause the newer battery to drain as quickly as the old one. Always buy marine-quality batteries. Automobile batteries will work, but they are not made to withstand the pounding and moisture that boat batteries must endure and won't last as long.

3. Pick a spot that is out of the way, but easily accessible, to mount your battery switch. A good place is under a rear seat. Another popular spot is under the steering wheel; however, you will need to run wires further to the batteries. Mark the screw spots and drill pilot holes for the switch. Cover the drill holes with marine sealant and then place the unit over the area and screw it on tight. Run the connection wires along the side of the boat down to the engine compartment where the batteries are stored.

4. Install the second battery next to the original one in the engine compartment. Secure with a strong strap or bungee cord. Choose which battery will be your "starter" battery and which will be your "house." The "house" battery runs all of the appliances on board, while the "starter" runs all engine and running requirements such as navigation lights. The usual choice for the starter battery is "A" or "1." Always use the same setting for your engine starter,and the same setting for your house battery so you never forget or get them confused.

5. Connect the positive (red) port of the first battery to the "A" or "1" switch. Connect both negative (black) ports to the engine as a ground. Connect the positive (red) port of the second battery to "B" or "2." Connect the alternator wire to the selector switch. Connect the negative (black) port on the first battery to the distribution panel. Connect the selector switch to the circuit breakers.

References

• Continuous Wave: Dual Battery Wiring
• Jerry Labella: Battery Switch, Isolators, and Integrator Systems How To