How to Wire a Boat With Both 12 Volt & 24 Volt

How to Wire a Boat With Both 12 Volt & 24 Volt

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Small boats usually use a 12-volt battery to power electrical systems, the same as most cars. However, the energy requirements on a boat can be different, especially if you install more powerful electrical items on your boat that need increased voltage. The simplest method to overcome the issue is to wire two 12-volt batteries in parallel, as this doubles the voltage, but also wire a switch between the two batteries, so you can choose to use 12 or 24 volts.

Items you will need

  • 12-volt battery

  • 3 battery cables (spring-loaded clamps on each end)

  • Wrench

  • Switch (suitable for use with battery cables)

Step 1

Open the boat’s battery compartment. Place a second 12-volt battery next to the existing 12-volt battery. You need to ensure the battery is the same size and produces the same amperes as the existing battery to get the best results.

Step 2

Connect a battery cable clamp onto the “+” terminal of the existing battery. Spring loaded ones are the best as you can open the jaws wide enough to go over the battery cable that’s already connected to the battery. If the clamp is using a nut and bolt then you need to loosen the bolt on the clamp that is attached to the existing battery using a wrench then push the clamp down hard using your hands so it makes enough room for you to connect the other battery cable clamp. Tighten the existing clamp then place the other clamp over the terminal and push it as far down as you can then tighten the clamp.

Step 3

Connect the opposite end of the battery cable to the “+” terminal of the second battery using the spring-loaded clamp or a wrench as appropriate.

Step 4

Connect a second battery cable clamp onto the "-" terminal of the existing battery. You need to use a battery cable with spring loaded clamps at each end for this part of the task.

Step 5

Connect the opposite end of the battery cable from the "-" terminal of the existing battery onto the terminal on a switching unit. Connect a third battery cable to the other terminal on the switching unit. Attach the opposite end of the third cable to the "-" terminal on the second 12-volt battery.

Step 6

Move the lever or switch onto the “On” position to link up the two batteries and produce 24 volts. Move it to the “Off” position to return to 12 volts.


  • The reason the switch is placed in line between the "-" terminals rather than the "+" terminals -- which you would usually do if you were wiring switch at home -- is because the negative cable from the existing battery connects to the metal parts of your boat and so it’s important to can change the voltage that flows through the boat. Permanently connecting the “+” terminals has no effect until you turn on the switch and complete a circuit.
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