A battery selector switch decides which battery feeds the engine and the boat: battery 1, battery 2, both, or neither. Here is how one is wired to two batteries, following the switch maker's own diagram, and how to use it without damaging the charging system.
⚓960×540 · boat-battery-selector-switch.webp
One switch chooses which battery runs the boat.
What a 1-2-Both-Off switch does
The switch has three terminals: one for each battery and a common one, marked COM, that is the switch's output. In Perko's diagram COM runs to the engine's starter; check your boat's own wiring diagram for how its other loads are fed. The knob connects COM to battery 1, battery 2, both batteries (Perko marks this position ALL), or nothing (OFF).
With one battery, a simple on-off switch does the same job: it sits in the positive cable between the battery and everything it feeds.
Each battery feeds its own terminal; COM feeds the starter.
Which cable goes where
From Perko's Diagram #1 for a single engine with two batteries.
Cable
Connects to
Battery 1 positive
Switch terminal 1
Battery 2 positive
Switch terminal 2
Switch COM terminal
The engine's starter
Battery 1 and battery 2 negatives
Tied together to engine ground
How to wire a dual battery switch
Disconnect both batteries
Turn everything off and remove both batteries' negative cables before you touch a positive one.
Mount the switch close to the batteries
Perko says to locate the switch so the battery cables are as short as possible. Its switches mount with #10 screws; don't overtighten them, or the switch body can deform.
Connect the battery positives
Run battery 1's positive cable to terminal 1 and battery 2's to terminal 2.
Connect the common terminal
Run a cable from COM to the engine's starter, as in Perko's diagram.
Tie the negatives together
Connect both battery negatives to engine ground.
Tighten and protect the terminals
Perko's switches use 5/16 inch studs, with the nuts torqued to 10 ft-lb. Where the switch is exposed to corrosion, Perko recommends a liquid electrical coating on the terminal connections.
Keep the automatic bilge pump off the switch
BoatUS says to connect an automatic bilge pump directly to the battery, with a fuse in its positive wire as close to the battery as possible, so it keeps working when you turn the power off and leave the boat. See the bilge pump wiring guide for that circuit.
Reconnect and test
Reconnect the negatives, then check that the engine starts and the loads work in positions 1, 2 and BOTH.
Who should wire it
Perko's instructions say the wiring to the switch should be done by a qualified marine electrician, to the NFPA 302 fire protection standard for motor craft, the American Boat and Yacht Council's standards and the Coast Guard's electrical rules for boats (33 CFR 183).
How to use a boat battery switch
Never switch to OFF with the engine running. Perko's instructions say to always stop the engine first. Turning a running engine's switch off can damage the alternator; Perko sells an alternator field disconnect version that protects the alternator's diodes if it happens by mistake.
Switching between 1, BOTH and 2 is fine with the engine running on a make-before-break switch like Perko's, which connects the new battery before it drops the old one.
Turn it OFF when you leave the boat. A correctly wired automatic bilge pump stays powered because it bypasses the switch.
If the switch acts up after sitting for months, Perko says to turn it back and forth briskly several times with no load connected.
Cable size and fuses
Perko rates its 8500 series switches at 250 amps continuous and 360 amps intermittent. Size every cable for the current it carries and the length of the run, and protect each positive conductor with a fuse or breaker sized for that conductor. For branch circuits, the marine wire gauge chart covers both wire size and where the fuse goes.
Common questions
Neither battery. COM is the switch's output, and in Perko's diagram it runs to the engine's starter. The battery positives go to terminals 1 and 2.
Between 1, BOTH and 2, yes, on a make-before-break switch such as Perko's. Never turn it to OFF with the engine running.
It connects both batteries to the common terminal at once. Perko labels this position ALL.
The automatic circuit should not. BoatUS says to connect an automatic bilge pump directly to the battery, with a fuse close to the battery, so it runs when the switch is off.
Where it keeps the battery cables as short as possible, according to Perko.
Perko torques the nuts on its 5/16 inch studs to 10 ft-lb. Check your switch's instructions.
Specifications come from manufacturers' published documents and can change. Check your engine or boat maker's current manual before you change a setting or fit a part.