I've installed an extra 12V source in my car and everything is working.

While installing it I removed the positive terminal from the battery and left the negative connected. (Resting the positive on the plastic cover on top of the terminal) Once the work was done I just reconnected the positive side of the battery.

I've been reading up saying there could have been issues if I had accidently touched the spanner to a metal part of the car and created a short circuit. I didn't have any issues (Didn't touch any metal with the spanner) and the only thing that happened was the terminal sparked a bit while I was pushing it down while re-connecting everything.

The car is all working but I just wanted to make sure it wouldn't have any adverse effects on the electronics in the car as I've read I was meant to remove the negative terminal first, then positive; reconnecting the positive first.

(which i obviously didn't do just disconnecting the positive while doing work)


2 Answers 2


Ah yes, those pesky little battery terminals. They ain't the same. The one labelled "+" (usually red) is connected to everything electrical in the car, while the one labelled "-" (usually black), only goes to a couple of bolts here and there. It would make sense, one would think, to disconnect the one connected to all the stuff, wouldn't it?

Well, it doesn't. Good practice is actually to disconnect the negative one for the following reason:

  • when the positive terminal is disconnected, it will arc when it comes into contact with just about any unpainted metal part of the car. The terminal doesn't have to touch a metal part directly, it can (and will) go through metal tools (like a spanner, screwdriver or pliers). The reason this arcing occurs is that, as stated above the negative terminal is connected to the body of the car (through a few bolts). Therefore, touching the positive terminal to the body of the car completes the circuit.
  • when the negative terminal is disconnected, it can't arc, only the negative battery post itself can (that's where the juice is coming from).

As for the sparks you saw, those are normal when reconnecting a battery and the battery charge has changed a little bit since the connection was broken. This does no harm to the battery.

  • Thanks for confirming that! I'm guessing as I didn't have any arching while taking the positive off and didn't touch the spanner to any metal part of the car while removing it that it's all okay then. I was just a bit concerned as I read up after I should have removed the negative lead then positive and just left both the leads disconnected while working on the car. I just wanted to make sure that from an electrical standpoint, connecting the wires either way round won't affect any electronics in the car. From a safety standpoint I now know to remove the negative first and re-attach last. Feb 13, 2017 at 15:32
  • 1
    Removing the negative terminal is so much safer! my tools are all metal. and I almost always bump the tool to something else when removing a battery clamp... If you always remove the negative terminal first, you will never have any issues. And remember first off = last to go on when replacing your battery. .
    – zipzit
    Feb 13, 2017 at 16:09
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    I know it's late to the show but the reason for disconnecting the negative terminal first is that IF your spanner or screwdriver touches the bodywork while it's connected to the terminal it doesn't have any potential difference and won't make a spark. If you work on the +ve terminal and touch the spanner to metalwork of the car body you have just created a short circuit which will spark and will discharge the battery through your spanner. Which will get very hot. Once either battery terminal is disconnected you can't make a circuit by touching either terminal to the car.
    – houninym
    Dec 4, 2019 at 12:16

I think the other answer is a bit misleading when it talks about arcing,

The battery positive terminal is usually connected to the fuse box via a thick cable and the starter motor by another thick cable.

The battery negative terminal is usually connected to the car’s bodywork by a single thick cable.

If the positive terminal is being disconnected first using a spanner/wrench the tool is then electrically connected to the positive terminal. If you accidentally touched the bodywork or any other metal part with the tool at the same time you will cause a short circuit capable of carrying hundreds of amps which can literally melt the tool, cause a fire and burn your hand.

If the negative terminal is being disconnected first using a spanner/wrench the tool is then electrically connected to the negative terminal. If you accidentally touched the bodywork or any other metal part with the tool at the same time you will only be connecting to the negative terminal of the battery, so no danger. As long as you don’t touch the battery positive terminal at the same time you will be OK and since the battery positive terminal usually has a red cover on it there is little risk.

Once the battery negative is disconnected, accidentally touching the battery positive to the cars bodywork using a tool is no longer dangerous.

If only the positive terminal is accessible at first, then you just need to be extra careful,

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