What was found
DURING ENGINE START WITH GROUND POWER UNIT AND AVIONICS ON, ALL 4 DISPLAY UNITS WENT BLACK AND THEN LIT UP AGAIN AFTER ENGINE STARTED. MAINTENANCE DISCOVERED WITH A CAPACITY CHECK ON THE EMERGENCY BATTERY THAT 1 CELL HAD FAILED WHICH CAUSED THE VOLTAGE TO DROP BELOW 22 VOLTS UNDER A HEAVY LOAD. EMERGENCY BATTERY REPLACED. EMERGENCY BATTERY IS AN ON CONDITION ITEM WITH A CAPACITY CHECK EVERY 3 MONTHS. AS THIS BATTERY WAS OVER 5.5 YEARS OLD WE WOULD RECOMMEND A LIFE LIMIT OF NO MORE THAN 5 YEARS PUT ON THESE BATTERIES.