As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING PRE-FLIGHT INSP, AIRCREW PERFORMED EMERGENCY LIGHTS OPS CHECK AND THEY NOTICED THE RED LIGHTS WERE VERY DIM. MX GROUND CREW VERIFIED THE PS4 BATTERY PACK VERY WEAK. BATTERY PACK WAS R & R, OPS CHECK PERFORMED. SYS PERFORMED SATISFACTORY. ACFT WAS RETURNED TO FLIGHT STATUS.