Q: A friend of mine told me that I should always disconnect the positive terminal of the battery first to keep the electricity from spilling out of the battery. I can't see how this would matter. I just unhook whichever terminal is closest first. I've never noticed any spilled electricity, and I think he's just pulling my leg.
A: Let me relate something that happened to a friend of mine, a dentist by trade who likes to work on his sports car occasionally. In preparation for some other work, he elected to remove the battery from the car, using a box wrench to loosen the battery clamps. The wrench made contact with his wedding ring (he's lefthanded), and the ring in turn made contact with the battery holddown. A massive short circuit welded his ring to the holddown and to the wrench, which in turn was welded to the positive terminal.
Car batteries store an enormous amount of energy, and they're optimized to deliver it in a very short period of time. A shorted car battery can easily deliver several hundred amps -- more than an arc-welding machine. Within a second, my friend's wedding ring was almost red-hot, and only a fast reaction with his other hand to break the connection by hitting the wrench prevented it from remaining connected long enough to melt. The ring had to be sawed off his finger at the emergency room, and it was more than a month before the burn specialist was sure he wouldn't lose the finger altogether.
Two lessons here. First, remove all jewelry when working with tools, even something as simple as a box wrench. That's fairly obvious, eh?
Second, always remove the battery's ground clamp before loosening the positive. If you remove the negative clamp and inadvertently complete a circuit to ground, there will be no current flow because the ground clamp is already grounded. Subsequent shorting of the positive terminal to ground will then produce no current flow because the current has no return path to the negative post. And, of course, always reconnect the ground last.
