C172 PLT HAD BURNING ODOR IN COCKPIT.

Narrative

WHILE VFR RETURNING TO UNITED STATES FROM CANADA I WAS SOLO OCCUPANT OF MY C172. AFTER APPROX 1 HR ENRTE I SMELLED SMOKE. AFTER SEVERAL ATTEMPTS TO LOCATE IT I DISCOVERED THE SOURCE TO BE A COTTON STORAGE BAG ON THE REAR SEAT OF MY PLANE. THERE WERE NO FLAMES COMING FROM THE BAG; JUST SMOKE. THE BAG WAS SEAT BELTED IN PLACE AND I WAS ABLE TO REMOVE IT AND SMOTHER THE SMOLDERING BAG. NO DAMAGE OCCURRED TO THE ACFT. THE FLT CONTINUED UNEVENTFUL. AFTER LNDG I DISCOVERED THE SOURCE OF THE FIRE. I HAD STORED AN EXTRA 9 VOLT BATTERY IN MY ACCESSORY BAG. THE BATTERY HAD BEEN REMOVED FROM ITS PACKAGING CASE IN CASE IT WAS NEEDED FOR MY HEADSETS (SPARE READY FOR USE). THE UNPROTECTED BATTERY TERMINALS HAD COME IN CONTACT WITH THE ZIPPER OF THE COTTON ACCESSORY BAG; SHORTING THE BATTERY AND EVENTUALLY CREATING ENOUGH HEAT TO BURN THE FABRIC AROUND THE METAL ZIPPER. I WAS VERY LUCKY TO HAVE HAD READY ACCESS TO THIS BAG. THIS SIT WAS EASILY PREVENTABLE BY LEAVING THE 9 VOLT BATTERY IN ITS ORIGINAL PACKAGING AND/OR USING A TERMINAL COVER ON THE BATTERY CONTACTS. I BELIEVE THE BATTERY WAS AN ALKALINE; INDUSTRIAL BRAND. THIS SIT COULD HAVE BEEN DISASTROUS IN THE BAGGAGE COMPARTMENT OF ANY ACFT; PVT OR COMMERCIAL.