As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: DURING PS CHECK OF EMERG LIGHTING STEP 27, ENCOUNTERED A "EMERGENCY POWER SUPPLY UNIT BATTERY AFT LEFT" MESSAGE. BITE AT EMERGENCY POWER SUPPLY UNIT BATTERY PACK 13WL IN AFT GALLEY CEILING SHOWS "BATTERY" FAULT. AFT GALLEY CEILING PANEL OPENED FOR ACCESS. REMOVED AND REPLACED EMERGENCY POWER SUPPLY UNIT BATTERY PACK 13WL.