Theoretically, nothing. The reason why jump start procedures never recommend connecting the negative directly to the battery terminal of the jumped car and instead to a good ground in the engine or chassis is because it minimizes the chance of arcing when you disconnect the jumper cable after starting thus preventing the spark from igniting any combustible gas released by the battery (it's hydrogen) and causing an explosion.
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Monday, April 29th, 2013 AT 5:58 PM