When changing a car battery, why do you disconnect the negative first and then the positive, and vice versa when putting a new battery in? What will happen if you do it the opposite way?

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When changing a car battery, why do you disconnect the negative first and then the positive, and vice versa when putting a new battery in? What will happen if you do it the opposite way?

In: Engineering
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If you’re cranking away with a wrench, you could accidentally bridge between the positive terminal and the frame. *boom lots of sparks!* If you disconnect the negative terminal first, you’ve isolated the frame from the battery.

The negative terminal is connected to the entire frame of the car, allowing you to use any metal point inside the car as a battery negative connection point. This means you could accidentally touch the positive battery terminal with a part of the car with your tools and cause a short circuit of epic proportions.

Disconnect the negative terminal first to break this connection, and now the only way to make a short circuit is to touch both battery terminals together which is much less likely to be done accidentally.

(There are a variety of reasons to make the entire car frame effectively the battery negative terminal, but that’s a story for another day)

The reason I was told to do it that way was due to the off gassing of lead acid batteries which have a low flash point meaning all those sparks you would get otherwise could ignite the fumes

So to answer the second half of the question, what will happen if you do it the opposite way… Unless you touch the positive terminal to something metal that’s grounded, absolutely nothing will happen. You’re breaking the circuitb either way. You can realistically take either cable off first and it won’t matter.

The idea of taking the ground off first is mostly just to prevent arcing if the positive terminal accidentally touches something. That’s it, plain and simple. Don’t over think it.