As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE PERFORMING EMERGENCY LIGHT SYSTEM TEST ON CIDS, GOT FAULT MSG EPSU BATTERY AFT L (ATA 33-51-38). REPLACED AFT LEFT EMERGENCY POWER SUPPLY AND BATTERY FOR 13WL UNIT. ACP POWER SUPPLY BITE AND AC EMERGENCY BATTERY CHECK BOTH TESTED GOOD REF AMM 33-51-38-000-001-A.