Before you do that then, take a piece of cloth or something and put it under and on the sides of the pack as a temporary insulator try it. If there aren't any more sparks, then that's what it was.
Now that makes a lot of sense, thats. Not cloth though, I was thinking about electrical tape around the contacts of the batteries. Also I think electrical tape will be a good insulator.
Any other suggestions or does that seem to be the problem.