As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: WHILE PERFORMING BITE TEST OF EMERGENCY LIGHTING SYSTEM, RECEIVED FAULT MESSAGE EMERGENCY POWER SUPPLY UNIT INTERNAL LOAD AFT LEFT. MAINTENANCE REPLACED BATTERY FOR AFT EMERGENCY POWER SUPPLY UNIT PER AMM 33-51-38. A SATISFACTORY OPERATIONAL CHECK WAS ACCOMPLISHED.