This is an old saying that always annoys me! People swear by it but I just don't see how it can be unless the theory has been distorted over the years. Could it be that just the coldness of the floor weakens the battery? As it goes if you put a car battery on a cement floor it will discharge so if it has to sit on the garage floor you want to set it on a board. I ask where is the conductor that provides a short between the poles? An electrician once told me it was just absolutely true although he didn't know how. Sounds like a wives tale or superstition to me.