A MECHANIC REPORTS ON INSTALLING A B737-800 APU BATTERY IN A B757-200 AIRCRAFT. BOTH BATTERIES LOOK IDENTICAL AND ARE FUNCTIONALLY EQUIVALENT TO EACH OTHER; BUT THE PART NUMBERS ARE NOT.

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I WAS ASKED BY MY LEAD TO REPLACE AN APU BATTERY ON ACFT. WE WERE VERY BUSY AND HAD A LOT OF WORK TO DO. I WAS WORKING ANOTHER ACFT AT THAT TIME. I STOPPED AND WENT TO THE STOREROOM AND GRABBED THE BATTERY I THOUGHT WAS CORRECT. I PULLED UP THE REMOVAL AND INSTALLMENT MAINT MANUAL PAPERWORK. I PULLED THE APU CIRCUIT BREAKERS AND OPENED THE BATTERY ACCESS PANEL. I PULLED THE BATTERY OUT ON ITS TRACKS SO I COULD SEE THE MANUFACTURER'S PART NUMBER. I COMPARED THE OLD BATTERY MANUFACTURER'S PART NUMBER WITH THE NEW MANUFACTURER'S PART NUMBER ON THE PARTS TAG. THE DASH NUMBERS MATCHED; SO I REMOVED THE OLD BATTERY AND INSTALLED THE NEW BATTERY PER THE MAINT MANUAL. I PUSHED IN ALL THE CIRCUIT BREAKERS AND CLOSED ACCESS PANEL. I WENT UPSTAIRS TO THE COCKPIT AND STARTED THE APU. IT STARTED JUST FINE AND CHKED OUT FINE. I CHKED THE ELECTRICAL PAGE. THE VOLTAGE OUTPUT WAS GOOD AND THE BATTERY STARTED TO RECHARGE AFTER THE START. I FILLED OUT THE LOGBOOK PAPERWORK AND PARTS TAG. I HAVE BEEN UNDER A LOT OF STRESS LATELY. I KNOW THERE IS NO EXCUSE FOR MY MISTAKE; AND I WILL DO MY BEST NEVER TO LET THIS HAPPEN AGAIN. THE MISTAKE WAS FOUND BY ANOTHER EMPLOYEE IN ZZZ. HIS JOB IS TO CHK PART NUMBERS TO ACFT CONFIGNS. HE HAD THE ACFT STOPPED AND THE BATTERY CHANGED.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THE B757-200 AND THE B737-800 APU BATTERIES ARE IDENTICAL; BUT THE PART NUMBERS ARE NOT QUITE THE SAME. THE FIRST TWO AND LAST TWO NUMBERS ON THE BATTERIES ARE THE SAME; BUT THE CENTER NUMBERS ARE NOT. REPORTER STATED SINCE BOTH AIRCRAFT APU BATTERIES ARE IDENTICAL IN LOOKS AND FUNCTIONAL; HIS CARRIER IS NOW WORKING TO DEVELOP A SINGLE PART NUMBER; OR COMMON LISTING FOR USE ON BOTH AIRCRAFT TYPES. REPORTER STATED THAT FOR THE TIME BEING; AN ADDITIONAL INFORMATION NOTICE TO MECHANICS HAVE BEEN ADDED TO THE APU BATTERIES PLASTIC TAG ENVELOPE; ALERTING MECHANICS TO BE AWARE OF THE CURRENT DIFFERENT PART NUMBERS.