Sidecar is exactly right. Electricity will always take the path of least resistance and if your ground is on one end and you are welding on the other, the electrical wiring is fair game to provide a path...through some widgets that will fry from a few amps much less than a welder pushing possibly a hundred or so. That being said, if I were to weld on a FEL and could attach the ground cable within a foot or so on some clean metal that I cleaned with a grinder, I would not bother to disconnect the battery. I have done this many times on vehicles and never had an issue.