Someone came here the other day looking for the answer to this question – which battery terminal should you connect first.  It’s a great question.  We’re talking about automotive batteries, and whether you connect the positive terminal or the negative terminal first.

You’ve got the hood open to your car, you’ve just done some electrical work of some kind and the batteries disconnected.  Now it’s time to reconnect them, and you want to make sure you continue to work with the best and safest practices.

The Short Answer

The short answer?  Connect the positive first and the negative second.  This way you eliminate the risk of creating a short while you’re connecting them.  If you want to be sure then read below for the caveats – there are situations where this doesn’t apply and/or you can damage equipment.  In the meantime, for most of us, here’s an explanation of why:

Short Circuits in Car Batteries are BAD!

Imagine you’ve connected the negative first, and now you’re on to the positive side.  You slip the battery clamp over the positive terminal.  Maybe there’s a tiny spark as your radio memory draws that tiny bit of power.  But no worries, that little spark has so little current running through it that nothing happens.  Next you take your half-inch wrench or 12 mm or whatever it is wherever you live.  You start tightening the clamp tightly to the post.  As you turn the wrench around it comes close to the body of the car and suddenly the wrench slips off the nut and slams against the body while still touching the the positive terminal.

Don’t try this at home kids!  If a circuit is completed (it might not due to the insulative properties of the car paint) but if the circuit is completed you’ll have a great spark show on your hands, and the point of greatest heat will loose.  You could melt a hole in your car body, you could melt the battery post (most likely being the softest metal), you could melt and deform your wrench, and of course you could burn yourself.

For most of us, I think we can agree this situation sucks.  If you had not connected the negative terminal first, then the body would not be connected to the battery system, and the short would not have been possible.

Connection Sequence of a Car Battery

The car body is connected to the negative terminal, the negative terminal is connected to the positive terminal (and in between has harnessed the electrical potential in the battery), the positive terminal is connected to the electrical system, and each item in the system is connected to the car body.  This is a complete circuit.

In the short circuit example above you actually “shortened the circuit”.  You cut out the electrical system, and just connected the battery straight to the body.  If the negative terminal was not connected, then the circuit would have remained incomplete, and you would not have had a short.

Clear?  If not, ask in the comments section below and I’ll try to clarify.  And the moral is connect the positive first, then connect the negative. Go ahead and slip with the wrench all you want when connecting the negative – it’s already connected to the car body, and you won’t create a short circuit.

The Caveats

Here are a few reasons why you might connect the positive terminal first rather than the negative terminal.

1. You’ve got a positive ground vehicle.  Like an older Series Land Rover – yes you know who you are!  The electrical systems in those cars are backwards to most of the rest of the cars in the world.  Yes, the English had their reasons.  If you’ve got a positive grounded car, then just do it in the opposite way.
2. You’ve got equipment that needs the ground to be connected last.  There are some pieces of specialized electronics (poorly made I’ll admit) that need to be connected in a certain order or they blow internal fuses.  If you’ve got them, you probably know all about it and you also probably curse yourself for not spending a bit more on a better engineering product!

Any more?  Leave a comment below if you know of any!

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