As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS NOT GROUNDED: ATA335100 BAT/EPSU (152WL) MID L CLASS 1 MESSAGE DISPLAYED DURING SYSTEM TEST. MAINTENANCE REPLACED BATTERY FOR EMERGENCY POWER SUPPLY UNIT 15WL PER AMM 33-51-38 AND ACCOMPLISHED A SATISFACTORY OPEPERATIONAL CHECK WITH NO FAULTS NOTED.