How to wire a boat battery switch

Written by mick wolff
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Adding a boat battery switch to your boat's wiring is an easy way to enhance your vessel's safety. A boat battery switch helps manage two batteries or battery banks. It allows you to engage these batteries separately or in combination and enables you to to isolate the batteries from your vessel's electrical system. The battery switch lets you shut down the vessel's electrical system quickly in an emergency or when leaving the boat for an extended period. It also lets you keep a starter battery in reserve in case the vessel's main battery runs down.

Skill level:
Moderate

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Things you need

  • Battery switch and mounting screws
  • Battery cables
  • Terminal lugs
  • Basic hand tools (wrenches, screw drivers, utility knife)
  • Crimping tool (i.e. Nicopress #3 Swage-it)
  • Optional: two high-amperage fuses and fuse holders

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Instructions

  1. 1

    Find a location for the switch. Keep in mind that the cables will run between the batteries, the switch, the engine, and any distribution panels or fuse boxes and that they should be as short as possible. The switch should also be protected from spray and curious guests, yet still be within the operator's easy reach.

  2. 2

    Determine where to run the cables between the switch and the batteries. You'll need to run two positive cables (one for each battery or battery bank) to the switch.

  3. 3

    Lay the cables in place and cut them to length, one at a time. Strip about 1/2 inch of insulation from the ends of cables and slip on the proper-size cable lugs. Crimp the lugs.

  4. 4

    Secure the cable lugs to the input terminals of the battery switch. The typical battery switch has three terminals: two battery input terminals and one output terminal.

  5. 5

    Determine where to run the cables between the switch and your boat's engine and electrical systems, depending on the complexity of your boat's wiring. You may only need to run a cable to your engine's starter motor solenoid, or you may need to run one to your fuse or distribution panel as well.

  6. 6

    Lay the cables in place and cut them to length, one at a time. Strip about 1/2 inch of insulation from the ends of cables and slip on the proper-size cable lugs. Crimp the lugs.

  7. 7

    Secure the cables to the output terminal of the battery switch. Mount the battery switch using mounting screws. Secure the cables to the engine's starter motor solenoid and distribution panel.

  8. 8

    Switch the battery to "off." Attach the negative terminals of the batteries to the boat's ground; for example, one cable running to the engine block and another to the negative wire coming from the distribution panel. Again, make sure the cables are properly sized, crimped and secured.

  9. 9

    Finally, you are ready to attach the cables to the positive battery terminals. Before taking this final step, however, consider installing fuses between the positive battery terminals and the cables that run to your new battery switch. High amperage fuses are now available at reasonable prices. They will protect your wiring (and perhaps your life) while still allowing you to draw enough current to start your engine.

Tips and warnings

  • Attaching three or more heavy cables to a single terminal on the switch (for instance, you may also have an electric windlass) may not be possible--thus it may require wiring the switch to a external distribution post first.
  • The proper size of the cable depends on the length of wire running from the batteries to the engine and distribution panels. Consult the tables published by the American Boat and Yacht Council to determine the proper gauge of the cables for your installation. Keep in mind that a typical starter motor draws as much as 160 amps.
  • Although a battery switch allows you to disconnect your boat's electrical system from the batteries, some things should not be disconnected. Bilge pumps, for example, should be properly fused and connected directly to the batteries. The last thing any boater wants is their boat slowly filling with water while tied to dock simply because their diligent use of a battery switch had also turned off the bilge pumps.

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