AN MD90 WAS DISPATCHED IN NON COMPLIANCE WITH 1 OF 3 INERTIAL REF UNITS BATTERIES REPLACED. MAINT MANUAL REQUIRES ALL BATTERIES TO BE REPLACED AS A SET.

Narrative

I WENT TO ASSIST 2 OTHER MECHS AND A LEAD MECH ON A PROB WITH AN ACFT IN SVC AT THE GATE. THE PLANE LOADED WITH PAX AND CREW FOR REVENUE FLT AND WAS ON DELAY. THE CREW HAS RPTED INERTIAL REF UNIT #1 BATTERY FAILED DURING PREFLT. THIS ACFT HAS 3 SHIPS BATTERIES WHICH ALSO SERVE AS BATTERIES FOR THE INERTIAL REF UNIT. I MEASURED VOLTAGE ON EACH ONE OF THE BATTERIES AND FOUND ONE OF THE 3 BATTERIES' VOLTAGE WAS EXTREMELY LOW. THE EXTREMELY LOW BATTERY WAS REPLACED BY ANOTHER MECH. I THEN WENT TO THE COCKPIT AND FOUND THE BATTERY CHARGER WAS NOT WORKING. THERE WAS NO SVCABLE CHARGERS IN STOCK. I ROBBED A CHARGER FROM ONE OF OUR LAYOVER ACFT. ANOTHER MECH REMOVED THE BAD CHARGER AND REPLACED IT WITH THE ROBBED PART. THE BATTERY; BATTERY CHARGER AND INERTIAL REF UNIT OPS CHKED NORMAL. I THEN CLRED THE ITEM AND SIGNED THE AIRWORTHINESS RELEASE. THE ACFT DEPARTED AND ARRIVED AT ITS DEST. THE NEXT DAY I WAS INFORMED THE ACFT WAS NOT FLYING BECAUSE THE CHARGER HAD FAILED AGAIN AND WAS AWAITING A CHARGER AND 3 BATTERIES. I WAS ALSO INFORMED THE BATTERIES ON THIS ACFT ARE CHANGED AS A SET AND NOT INDIVIDUALLY. I THEN READ THE MAINT MANUAL FOR THE BATTERY REPLACEMENT AND DISCOVERED ON THIS ACFT THE BATTERIES MUST BE CHANGED AS A SET. I THINK THE CAUSE OF THE PROB WAS NOT REVIEWING THE MAINT MANUAL FOR BATTERY REPLACEMENT AND STOCKING THE BATTERIES INDIVIDUALLY AND NOT AS A SHIP SET. TO PREVENT A RECURRENCE OF THE PROB; WE THEN SENT A LETTER TO OUR FOREMAN TO GET THE BATTERIES STOCKED AS A SET AND NOT INDIVIDUALLY. I INFORMED THE 2 OTHER MECHS AND THE LEAD MECH; WHO WORKED ON THIS ACFT; THAT THE BATTERIES ON THIS ACFT ARE TO BE CHANGED AS A SET. ALSO; THE MAINT MANUAL SHALL BE USED AT ALL TIMES EVEN IN A DELAY.