A Lesson Learned the Hard Way

Dec 12, 2015

RAY: This puzzler is from Gary from Santa Clara, who writes:

As a shade tree mechanic, I've read many times that when removing a car's battery or doing electrical work, one is supposed to disconnect the negative terminal first, and only then the positive terminal. Installation, as so many instructions say, is the reverse of that procedure.

As an electrical engineer, I scoffed at these instructions. I knew that opening a circuit at any point was equivalent to opening it anywhere else. I just disconnected whatever terminal was handy.

One day, I was helping a friend change the battery in his garden tractor. He happened to be an electrician, and he knew as well as I did that it didn't matter which terminal was removed first.

But, as we learned the hard way that day, this isn't true. It does matter which is removed and replaced first.

Why is that?

Answer: 
RAY: You're using a metal wrench to disconnect the battery cables. If your wrench contacts a piece of metal that's connected to the frame, you've got a short circuit, because all the ground circuits are connected to the frame, and the negative terminal of the battery is connected to the frame.

If you disconnect the positive terminal first and touch the wrench to a metal ground on the car, it’s tantamount to laying your wrench across the two terminals of the battery. So the reason that you disconnect the negative first is, if in the process of loosening the bolt to remove the negative terminal, the wrench should contact the frame, it doesn't make any difference. Because the negative terminal is already connected to the frame. And that's why when you go to put the battery back in, you reverse the order. You connect the positive first and then you connect the negative last.

Do we have a winner?

TOM: The winner is Monica Hartley from Houston, Texas. Congratulations!

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