[Old Henry]

I've jumped my 6 volt positive ground system from a 12 volt battery connected the same as you would two 12 volt batteries many times without any damage to anything. But it takes two people, one to clamp the jumper cables on the instant the other hits the starter and unclamp them as soon as the engine starts. Otherwise, the battery cables (and probably other things) heat up pretty fast. Gotta have extra heavy jumper cables too. The usual 12 volt flimsy cables might not work as well.