As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
REFERENCE LP541812 #1 DURING SCHEDULED HEAVY CHECK EPSU BATTERY 16WL FAILED TEST. AIRCRAFT WAS NOT GROUNDED. REMOVED AND REPLACED EPSU BATTERY 16WL IAW AMM 33-51-38-000-002-A/33-51-38-400-002-A OPS CHECK GOOD. AIRCRAFT WAS RETURNED TO SERVICE.