What was found
INTERIOR REAR EMERGENCY LIGHT ON LAV BULKHEAD & EXIT LIGHT BELOW LAV SEAT INOP WHEN USING BATTERY PACK. BOTH FUNCTION ON SHIPS MAIN BATTERY POWER. POSITIONED THE ACFT INSIDE HANGAR. APPLIED POWER & ALLOWED ACFT TO BOOT UP & VERIFIED THE WRITE-UP. REMOVED THE BATTERY PACK NR 4 DUE TO INTERNAL FAILURE. INSTALLED A NEW BATTERY PACK. PERFORMED OPS CHECK, & THE RESULT WAS SATISFACTORY. ALLOWED BATTERY TO CHARGE WHILE THE GPU WAS IN USE. RETURNED ACFT TO ITS ORIGINAL CONFIGURATION. DONE IAW AMM 33-50-06.