For Nikon F3/FM2/FM3A The DB-2 keeps your camera and metering system operating in extreme cold. Remove the two batteries from the camera battery chamber, attach the chamber to the DB-2 loaded with two AA-size alkaline batteries (do not use NiCd, Ni-MH or lithium batteries) via DB-2 connecting cord, then put the DB-2 in a coat or other warm place.