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Most of the advice I've seen for jump starting cars instructs to connect the black wire to a bare piece of metal on the car with the dead battery. However, I've always just hooked up both poles to the corresponding poles on the other car's battery. I've never experienced any negative consequences, and it has always worked fine.

Out of curiosity, does it matter, and if so, why?

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I wrote a series of blog posts on electricity, which addressed this (and the reasoning behind attaching the cables in a certain order) as a corollary. See How does jump starting a car work if current only flows when there's a difference in voltage? – BlueRaja - Danny Pflughoeft Sep 8 '11 at 17:25
up vote 31 down vote accepted

From an electrical perspective, it doesn't matter.

However, a lead-acid battery that is charging or discharging rapidly will give off hydrogen, which is highly explosive. Since you generally make the ground connection last, there's a good chance that you'll get a spark, which is enough to ignite the hydrogen.

So while it's unlikely that you'll have anything explode, under extreme conditions it's possible. Making the ground connection away from the battery eliminates the possibility.

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+1 for the mention of hydrogen. – Timo Geusch Jul 19 '11 at 20:51
Hydrogen has fanboys? Impressive for an element with so few protons. On the other hand, I guess the skinny elements are always most popular. – JohnFx Jul 19 '11 at 21:21
To clarify: the last connection should be made to a good ground point away from the battery so that sparks caused by making the connection do not ignite the hydrogen. – Les Feb 28 '14 at 20:08

Chris makes a very good point regarding the hydrogen given off by the battery. There's also another reason, especially on older cars. Ground straps to the engine on most cars are notorious for corroding, so by hooking up the negative terminal to the engine you'll increase your chance of getting the maximum current flow when you're trying to crank the engine.

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Are you saying that the black pole on the battery may not be fully connected to the chassis? – intuited Aug 22 '11 at 14:05
I'm saying that there might be some resistance due to corrosion between the negative pole on the battery and the various chassis grounds. – Timo Geusch Aug 22 '11 at 14:53

You run the risk of damaging the alternator and/or computer ECU on the Donor/Booster car, when connecting directly to the negative of the dead car.

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