WHILE ENROUTE FM FT LAUDERDALE, FL TO MONTREAL, CANADA, AIR CANADA FLT 087 REQUESTED TO MAKE AN UNSCHEDULED LNDG DUE TO SMOKE IN THE REAR OF THE ACFT. C-FTNE WAS VECTORED TO PATUXENT RVR NAS, PATUXENT, MD AND DESCENDED OUT OF FL 370. THE SMOKE WAS DETERMINED TO BE ORIGINATING FM A BLACK BOX IN AN OVERHEAD BIN LOCATED ABOVE SEAT #42. ATMPS WERE MADE DRNG THE DSCNT TO IDENTIFY THE BOX AND FIND THE APPROPRIATE CIRCUIT BREAKERS BUT WERE UNSUCCESSFUL DUE TO LACK OF LABEL ON THE BOX AND OTHER DUTIES OF THE CREW DRNG DSCNT. THE ACFT LNDD W/O INCIDENT AT PXT NAS AT 2306 HOURS EST ON MARCH 4, 1985, AND THE CPT RPTD THAT THE SMOKE STOPPED WHILE TAXIING TO PKG. INVESTIGATION REVEALED THAT THE SMOKE ORIGINATED FROM THE EMGCY PWR PACK WHICH CONTROLS THE EMGCY LIGHTING SYSTEM. A CASE TO CASE ELECTRICAL SHORT BETWEEN 2 ADJACENT CELLS IN THE SELF CONTAINED PACK RESULTED IN OVERHEATING. THIS WAS TRANSFERRED TO THE POLYETHYLENE TAPE USED TO HOLD THE 24 CELLS TOGETHER. AN INTERNAL FUSE IN THE BATTERY CHARGING CIRCUIT WAS FOUND POPPED.