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.
Another type of battery disconnect switch is this simple version that fits on the battery post, assuming you're running regular post terminals. No modifications are needed, though obviously you'll have to access the battery to operate it. This particular version is sold by SO-CAL Speed Shop. | 
This cheap master/disconnect switch was sourced at a swap meet for just a few bucks. It'll isolate the battery from the electrical system and can be mounted anywhere on the car, such as under the rear if your battery is located in the trunk, or even alongside the driver seat. However, it's likely to be a low-amp switch, which can cause resistance. | 
Speedway also sells this neat memory guard for your electronics. It connects across the terminals of a disconnect switch and contains a 3-amp fuse. This allows power to the accessories when the switch is thrown, but the fuse will blow if anyone tries to start the vehicle. |
Speedway Motors offer these disconnect switches, with an aluminum handle in red on the two-post version and a natural finish handle on the four-post. The four-post version is for use with an alternator and is required by the NHRA for competition use. They are rated at 180-amp continuous and 1,000-amp surge. | 
Flaming River's heavy-duty Big Switch is rated at 250 continuous amps and 2,500 amps surge for five-second intervals. It's available with a removable stainless key (as shown), a T-handle, or with a lever kit, accepted for IHRA/NHRA competition use. |  |
Not only does Flaming River's Big Switch have a three-year warranty, but as this cutaway version shows, it's spring-loaded, with large contact areas (arrow) on the actual switch mechanism. Because it is spring-loaded, compared to the rotary operation of most switches of this type, it ensures full contact immediately on turning the handle. | 
Again from Flaming River, this is a combination battery and alternator kill switch, and is designed to shut down the battery and electrical system, whether for security or in an emergency situation. This version features a large red button that simply has to be pushed to shut off the power. It's rated at 2,000-amp surge and 150-amp continuous flow. | |