What was found
BATTERY HALF ROCKER OF MASTER SWITCH BROKE IN HALF ALLOWING ELECTRICAL CONTACT BAR TO DROP OUT OF SWITCH BODY RESULTING IN COMPLETE LOSS OF ELECTRIC POWER. THIS FAILURE OCCURRED DURING GROUND OPERATIONS AT NIGHT. CAUSE OF FAILURE COULD NOT BE DETERMINED AS PILOT MISPLACED BROKEN PIECES. SUBMITTER SUGGESTS A CLOSE VISUAL INSPECTION OF CENTER AREA OF ROCKER FOR FINE CRACKS AT 100-HOUR INTERVALS FOR AIRCRAFT HAVING A HIGH NUMBER OF HOURS.