# Connecting the wrong jumper cable to car body?

Inspired by this thread: Every time I’ve had to hook up jumper cables, I’ve always hooked all the clamps directly to the battery terminals. The major reason for this is that, since I don’t hook up jumper cables very often, I’m never quite sure if it’s the negative or the positve that’s supposed to be connected to the car body, rather than the battery terminal.

So my question is: What would happen if I got it wrong? In other words, if I connect the two negative battery terminals, but then put one end of the other cable on the positve terminal of the good battery and the other end on the car body? It seems like I’d be making a short circuit; it would be the equivalent of connecting both cables to the negative terminal of the bad battery, wouldn’t it? Which is the same as connecting them to each other, which means that it’s the same as connecting the two terminals of the good battery to each, which is a short circuit. So not only would that not help the bad battery, it would potentially melt the cables, right? Is that the worst that could happen, or is there explosion possibilities?

Your concept is right. You just have to know that vehicles are negative ground*. Do what you have to do to memorize this, as the consequences of a mix-up can be dire. Your strategy of using the battery terminals is not a bad one. Although there is good reason to attach one clamp to a good ground point on the engine (preferable to the body) rather than to the battery terminal, the odds of an explosion are quite small. If they’re hooked up wrong as you describe, the odds of a problem are 100%.

The short circuit you describe will produce a big arc at the moment of connection, increasing the chance of battery explosion. If left connected, it will also make a heating coil out of the jumper cable. I would expect the insulation to melt and burn in short order - and the cable and clamp will probably be too hot to handle, making disconnection painful and injurious, if not impossible. The battery would probably be electrically damaged, and expensive electronic components such as the alternator and PCM (computer) could be ruined. Definitely worth avoiding.

*Some American cars into the 50’s and British cars into the 60’s used positive ground before negative ground became universal.

I once heard somone use the mnemonic “Minus to Metal”, which seemed as good a way as any to remember that cars are negative ground if you weren’t sure.