How to Install Two Battery Cut Off Switches for Boats
Two-battery cutoff switches allow you to wire two or more batteries into a parallel circuit. The switch will allow you to isolate or combine the two batteries. Typically, one battery is considered the house battery which runs all the DC loads on the boat such as radios, lights and navigation equipment. The other battery is the starting battery and be used for starting the main engine only. The switch allows the charging of one or both batteries at once, as well as allowing you to parallel the house battery with the starting battery to provide more cranking amps if needed.
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Things You'll Need
- Adjustable wrench (2)
- Steel brush
- Measuring tape
- 10 feet of 00 gauge red electrical cable
- 10 feet of 00 gauge black electrical cable
- Electrical cable cutters
- Razor knife
- 8 00 gauge cable lugs
- Lug crimper
- 2-pound hammer
- Battery switch
Instructions
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1
Remove the main power leads that feed the main power panel from the battery with the wrenches, negative side first, positive second. Ensure that the cable lugs are free of corrosion. Polish the lugs with a steel brush.
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2
Measure a five-foot section each of red and black cable, then cut with the cable cutters. Strip the last inch of each cable end. Insert the stripped end into a cable lug. Place the cable lug into the crimper. Strike the plunger of the crimper with a 2-pound hammer. Strike solidly and repeatedly until the lug is tightly crimped onto the cable. Repeat this process on all of the cable ends.
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3
Mount the battery switch onto a sturdy spot close to the batteries. Place the switch to within five feet of both batteries terminals. Mount the switch by the method provided or recommended by the switch manufacturer.
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4
Remove the battery bolts from the terminals on both batteries. Install a new five-foot red cable onto the positive terminal of the house battery and tighten the terminal bolt. Install the other end of that cable onto the "Battery 1 Positive" side of the battery switch and tighten the nut down firmly with a wrench.
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5
Install a new five-foot black cable onto the negative terminal of the house battery and tighten the terminal bolt. Install the other end of that cable onto the "Battery 1 Negative" side of the battery switch and tighten the nut down firmly with a wrench.
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6
Repeat this process for the starting battery, and connect the leads to the "Battery 2" lugs on the switch. Write a reminder on the mounting bracket that battery 1 is house and battery 2 is starting only.
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Tips & Warnings
When starting the engine, the switch may be set on "Neither" or "Battery 1." This keeps the starting battery isolated from the house bank. If the starting battery becomes weak and more amps are needed, put the switch to "Both" and you will draw current from both batteries. Turn off all lights and other house DC draws when paralleling the batteries. This will prevent wasting the extra battery power on unnecessary loads.
Be careful using both batteries to start the engine. If there is a problem that is causing a hard start such as clogged fuel filters, no fuel or a fuel/air leak, then more cranking amps will not do any good. You will only succeed in killing the battery that runs your navigation equipment and marine radio, things you will need to call for help and provide a location to rescuers.
