I have to credit reader John Fobian with the idea for this article, since he contacted me after reading our news item about Outta Sight's slick battery boxes. These come with what was described as a kill switch, though John quite rightly pointed out that there's a difference between a kill switch and a battery disconnect switch. So, what is the difference? A kill switch will shut down a running engine without damaging the alternator, while a master/disconnect switch may or may not kill the engine, its job being to isolate the battery from the rest of the electrical system while the engine is not running. However, this type of switch can damage the diodes in the alternator, as a high-voltage spike can be created when the switch is flipped. It should be wired with one terminal connected to the battery's positive terminal, and the other to the car's electrical system, with the alternator output connected directly to the battery positive.
A kill switch has six terminals, or more correctly, three pairs of terminals. The large pair connects one side of the switch to the battery and the other to the starter motor, a smaller pair will be for the ignition and fuse box feed, while the third pair will be for alternator surge protection.
Most of us have seen cheap master switches at swap meets, available for a few dollars. They're easy to identify since they only have two terminals compared to the six terminals on a kill switch, and so long as one is aware of their limitations, they can work effectively as a cheap security device. With the big red key removed, your car's electrical system is disabled, so long as a thief can't access the rear of the switch to jump the terminals, or doesn't possess his own big red key! However, these cheap switches won't have an amperage rating as high as a quality switch, which will typically be rated at 1,000-amp-or-more surge and 150-amp-or-more continuous flow.
According to the guys at Flaming River, a disconnect or kill switch can act as a choke point for engine performance, so it makes sense to choose one with as high an amperage rating as possible.
So, with safety and security in mind, we figured a quick look at what's available in both kill and master switches might be useful, as well as how to wire a kill switch into your electrical system. They may not be the most exciting product you'll purchase for your project, but you could well be thankful you fitted one, whether to save a battery from draining or to shut down an errant engine.
