看这个讨论

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]

所有跟帖： 

• I don't understand what causes spark if I just connect to negati -weiyi- ♂ (160 bytes) () 10/11/2012 postreply 08:47:39

• 讨论里都讲到了 -满地找牙- ♂ (261 bytes) () 10/11/2012 postreply 08:53:02

• 多谢, 那就照办了. -weiyi- ♂ (0 bytes) () 10/11/2012 postreply 09:06:42

• 是，就跟电池负极形成回路， 但回路的电阻值是不同的。 -我胡说八道- ♂ (0 bytes) () 10/12/2012 postreply 06:33:14