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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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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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    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, 2011 at 21:21
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    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, 2014 at 20:08
  • Note that in most cars the only easily accessible connection point away from the battery is to ground. So that is why it needs to be the ground connection. If you could find an accessible +12 connection with suitable ampacity you could connect there as well, but they are usually hard to get at.
    – dlu
    Aug 26, 2016 at 17:33
  • From an electrical perspective it may matter, due to the large currents involved. Anything you can do to reduce the resistance in the jumper cable circuit will serve to increase the current available to the starter. Shortening the length of the cables – moving the ground closer to the starter will do this. Not to say that you need to do this, but if you're not getting enough juice to start, it would be worth a try.
    – dlu
    Dec 22, 2016 at 18:34
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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, 2011 at 14:05
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    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, 2011 at 14:53
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The goal is to start the car (specifically spin the starter motor) not charge the "dead" battery. The good battery can be wired to provide power just to the starter motor since the starter motor is grounded to the chassis. If you connect pos-pos and neg-neg, the “dead” battery begins rapidly charging and Hydrogen gas is produced as a surge of electrons enters the dead battery, too rapidly for the dead battery to capture and store. You will not charge the dead battery if you connect the good battery negative clip to the chassis.

(I am not an electrical engineer (I'm a food engineer), so I may be wrong, but this concept has bothered me and I’ve never seen a decent answer, but I think this is the briefest and fullest true explanation, I hope!)

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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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