5.0 Mustang & Super FordsHow To Interior Electrical
Trunk-Mounted Battery Install - Power To The Project
Project Real Street Continues With A Trunk-Mounted Battery, Cables, And A Remote-Disconnect Switch
Horse Sense: NHRA rules mandate that a battery-disconnect switch be installed on the positive side of the battery. This is fine for a purpose-built race car without an alternator, but for a street car, the resulting voltage spike from the car's alternator when turning the switch can destroy all sorts of on-board electronic parts. M.A.D. Enterprises has the answer with its detailed electrical wiring schematics that isolate the alternator from the disconnect-switch circuit.
There's plenty of reasoning behind relocating your Mustang's battery to the trunk-increased weight transfer, more even weight distribution, more room under the hood, better looks, security, and so on. But just throwing your battery into the trunk and extending the battery cables is the wrong way to go about doing it. We could quote all sorts of laws and theories about why it's bad to wire the relocated battery like this and throw around terms such as "voltage drop," "Ohm's Law," "current flow," and "resistance," but only a few of you wattage-inclined enthusiasts would bother to continue reading. The rest of you would likely become bored and move on to the next horsepower-making article. Instead, we'll keep it simple and talk about safety and quality of the installation.
When it comes to mounting a battery in the rear of your Mustang, be it street or race oriented, you should keep a few things in mind. For starters, think about the weight of everything you're moving. A 30-plus-pound battery needs to be strongly secured via metal mounting straps. No rope and bungee cords here, as on your buddy's '76 F-150. And if you do any drag racing, you need to make sure your installation follows the letter of the NHRA or IHRA rule book, otherwise most racetracks won't let you run. While we don't have room to quote the full rules, you should know that besides the secure holddown, the battery must be fully enclosed in a plastic or metal box (which is to be vented to the atmosphere) and have a remote-disconnect system that interrupts the positive side of the electrical system.
We contacted Mark Hamilton at M.A.D. Enterprises about the battery relocation and wiring for our Real Street project. We've used M.A.D.'s products in the past on Editor Turner's '89 notch, and we fell in love with the quality and completeness of the company's kits. Mark had the same uneasy feelings we did about disrupting the positive side of the electrical system. He told us of his battles with NHRA to at least have the rule rewritten to use the negative side of the battery for the disconnect, which is much safer although still dangerous for voltage spikes in a standard street-car charging system.
When the NHRA wouldn't budge, Mark went back to his proverbial drawing board and designed a system that would indeed interrupt the positive side of the electrical system-but not interrupt the alternator-thereby eliminating any voltage spikes that could fry ignition boxes and computer components. We've incorporated his new design into the charging system wiring of our Real Street project. After you read this article, you'll want to change your trunk-mount battery setup to be this safe too.