There's the same amount of sparks but now instead of the sparks being on a grounded part of the engine away from the battery the sparking occurs right at the te

There's the same amount of sparks but now instead of the sparks being on a grounded part of the engine away from the battery the sparking occurs right at the terminals. The problem with bad batteries is that it might not just be dead, batteries will vent hydrogen and oxygen and on batteries that lost most of the electrolyte in a cell they might have enough hydrogen in them to blow the battery apart and throw acid over the person jumping the battery. Also, the risk of battery acid leaking is probably a good bit better than you might think. If you just grab a run of the mill car battery there's a very good chance that it's just a regular old wet cell battery with an open vent so it'll leak acid if you turn it upside down.