Q: My new battery disconnect switch has two large terminals, and two smaller terminals. How should I hook it up?

A: The large terminals should be connected (in series) to positive battery cable. This will protect sensitive electronic equipment in the event of a ground failure. The smaller terminals are used to shut off power at the alternator by opening (or breaking) the field circuit.

Some battery disconnect switches have only two large terminals. In this case, you would reroute the alternator charge output wire to the battery terminal side of the switch. This isolates the alternator and battery from the vehicle when the switch is in the off position. It’s also the best way to shut down a one-wire alternator system. The charge output circuit should be fused. Please remember to use a lower-gauge (thicker) wire on longer runs.

A note for racers: If your battery has been relocated, NHRA and IHRA rules require a battery disconnect switch to be mounted at the rear of the vehicle, and be accessible from outside of the vehicle. It must be wired in such a manner that halts all electrical function and kills the engine in the off position. The off position of the switch must be clearly labeled.

This is another in a series of weekly Q&A Mailbag sessions with Summit Racing‘s tech department, in which there are hundreds more. Click here to see them all.

Author: Dave Matthews

Dave Matthews was a mechanic for the U.S. Army, a Ford dealership, and served for many years as a fleet mechanic for construction companies. Now a technical content producer at Summit Racing, Dave has spent decades working on everything from military vehicles to high performance race machines.