Disconnecting the battery: Why, where & how
For some jobs it’s a great idea to disconnect the battery. Often it’s recommended for work on the fuel system (because if you disconnect the negative terminal it takes the risk of inadvertent electrical sparks pretty much off the table).
The way a car electrical system works: the metal structure of the car is connected to the negative terminal. All the wires are connected to the positive terminal. So, for example, the headlight has a wire that runs essentially back to the positive terminal on the battery. The circuit is completed via electrical contact with the body, which is in touch with the negative terminal. So, if you disconnect the negative terminal a wire that you disconnect during the job you’re performing can’t dangle and spark itself on the body, thereby causing a fire or blowing a fuse or blowing up the petrol pump you just disconnected.
Unfortunately, when you disconnect the battery negative terminal, the car’s audio system generally presumes it’s just been stolen. As a result it requires you to input a unique PIN code before it will work again. If you don’t know the code … that’s a real pain. Sometimes it’s almost impossible to figure out how to put the code back in, too, even if you know it.
For this reason, when car batteries die and need to be changed, the auto industry often employs a ‘code saver’ or 'memory saver' backup battery that you plug into your cigarette lighter, which keeps the electrical system alive so that the stereo doesn’t think it’s just been stolen.
Some well intentioned DIY-ers disconnect the battery as demanded by the job at hand, but put the code-saver in place first to minimize the hassle of re-coding the radio. Admittedly the risk of sparking is reduced, because the code-saver lacks the sheer electrical grunt of the car’s normal battery – but doing this can have unforeseen, unfortunate consequences.
For example, with the electrical system energized even by a tiny code-saver battery, the airbag deployment system is ready to rip. It requires very little current to operate. So, if you’ve disconnected the battery but put the code-saver in place, and then you go play DIY auto electrician and accidentally put some voltage into one of the car’s bright yellow airbag deployment wires, you’ll deploy the airbag. (good safety tip: all the car’s airbag deployment wires are bright yellow. It’s the law.)
Best-case scenario? Deploying an airbag accidentally will be expensive. Worst-case? Deadly. Best advice? Learn how to re-code your radio. Know the code. If the job at hand demands disconnecting the battery, don’t use a code-saver. Save to code-saver for battery replacement, which is what it was designed for.