[x factor]

One of the old Body men that I worked with years ago, also a shop owner saw a guy blow up a battery from having the battery connected. I would expect he probably was welding right next to it.
Now on the other hand I too have welded on alot of vehicles without disconnecting the battery with no problems, I just keep the ground close to what I'm welding. Newer vehicles I always disconnect due to all the senative electronics just to be safe with all the air bags and computer junk. That is usually major structural repair and the battery comes out more often than not just for access.