It depends on manufacturing and assembly tolerances -- most batteries in N800s and N810s fit snugly enough, but a few have excessive looseness -- folded paper, a little piece of tape, or whatever takes care of it. Honestly, I'd rather have a separate clip hold the battery in, but that's supposed to be the battery cover's job. Tape keeps mine from coming loose when I change cards, even though it has no perceptible rattle when closed.