What was found
WHEN CONDUCTING POWER ON CHECKLIST, EMER LIGHTS - CHECK (EMER LIGHTS - SWITCH TO ON), ONLY ONE OF THE THREE AIRCRAFT RIGHT EXTERIOR EMERGENCY LIGHTS ILLUMINATE. ALSO, INTERIOR SEAT EIGHT OVERHEAD EMERGENCY LIGHT DOES NOT ILLUMINATE. WHEN AIRCRAFT L AND R BATTERIES ARE THEN TURNED ON, AND ALSO WHEN APU IS ON AND APU GENERATOR IS ON , ALL EXTERIOR AND INTERIOR, EMERGENCY LIGHTS THEN BECOME ILLUMINATED AND FUNCTION PROPERLY. REMOVED AND REPLACED #4 EMERGENCY LIGHTING BATTERY PACK IAW CE-680 AMM 33-50-10. CHARGED BATTERY PACK OPS CHECK SATISFACTORY.