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The negative terminal is generally grounded to the chassis, which is why that would work. It's safer to connect it to the chassis because then the spark that may form won't be near the battery. If the battery is giving off hydrogen gas (unlikely, but it can happen), then such a spark could cause a fire or explosion. I think modern batteries don't vent that way, but that's only a vague recollection.

Oh, so there's no actual problem with clamping the negative onto the negative of the dead car battery in itself, but just the danger of sparks near the battery?
posted by subject_verb_remainder at 5:04 PM on December 29, 2011 [1 favorite]

It's the sparks that are the issue, yes.
posted by plinth at 5:07 PM on December 29, 2011 [2 favorites]

Right. The ground/negative of the battery is the "source" of the ground for the chassis. So the only reason to not hook it right to the battery is the supposed hydrogen explosion theory. And the time to worry about that is when disconnecting.
posted by gjc at 5:08 PM on December 29, 2011 [2 favorites]