What happens if you hook up negative first?

Answer 1

If the negative is already connected, when you hook up the positive side, it will spark. This spark can cause a fire is the there is any fuel vapor in the engine compartment or if the battery is leaking.

The negative side is less likely to spark when connected, so it is safe to make that your last connection.

Another Answer:

It does not matter, in terms of sparking, whether you connect negative or positive first. If there is going to be a spark, it will occur at the moment a circuit is completed, and that only happens when both are finally connected.

The issue of disconnecting negative first and reconnecting it last is simply one of protection from sparks (or more problematic, shorts) caused by the tool. If you put a wrench on a battery terminal and turn it, it is always possible that the wrench will touch something and cause a spark or short. In all probability, that something will be grounded to negative, so the greatest risk is when you disconnect positive first or reconnect it last. I always disconnect negative first, and reconnect negative last, so that, in case the wrench does touch something, it will touch ground and not complete a circuit, nor will it create a spark or short.

The other possible interpretation of the question involves the order in which you connect jumper cables. Normal practice is to connect positive to positive, and then negative to negative, but there is a critical distinction that some people miss... That last connection - the one that ultimately creates a circuit, should not be directly to the battery. Batteries, particularly run down batteries, create explosive gases, and creating a spark near them is bad.

What you should do is make the last connection to a solid frame ground, away from the battery. I usually connect to the alternator bracket, making sure, of course, that I don't interfere with the belt or pully. Frame ground is the same as negative, so making the connection there, while generating a spark, does so somewhat away from the battery, reducing the chance of it exploding.

There is something else I also do, that I recommend highly. When making that last connection to frame ground, I take my other hand and hold it between my face and the battery. That way, if for some reason the battery does explode, it won't spray sulfuric acid in my eyes.