Q: I’ve always jumped a car battery by connecting jumper cables directly from the positive and negative terminals on the good battery to the corresponding terminals on the other. However, I’ve recently been told not to directly connect the negative cable to the weak battery. Why not?
A: Technically, if you connect the negative cable to the negative terminal of the weak battery, the battery terminal could spark and ignite hydrogen gas surrounding the battery. For the sake of user safety, books and magazines such as Extreme How-To suggest you clamp the negative cable to the negative terminal of the good battery, and then clamp the other end of the negative cable to metal part of the automobile engine, such as the block, a bracket or bolt-head.