Just about all batteries lock to the frame somehow. The only problem is that the downtube ones are only held by 5mm riveted studs. Anybody that know that could wiggle one off with its holder in a few seconds, leaving you with holes in your frame; however, the battery can't be removed from the holder without the key, though I guess it would be possible to drill the lock in the comfort of home. It would then need a replacement lock before the battery could be used. All in all, probably not worth it to try and steel a battery unless you know how to repair them.