As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLIGHT 3559, CREW REPORTED, AFTER TAKEOFF AT AROUND 3000 FT HAD A MASTER WARNING BATTERY 2 OVERTEMP MESSAGE DISPLAYED ON EICAS. FOLLOWED AOM PROCEDURES AND DECLARED AN EMERGECNY AND RETURNED DEPARTURE. AIRCRAFT LANDED WITHOUT INCIDENT. THE AIRCRAFT WAS REMOVED FROM SERVICE. REPLACED THE BACK UP BATTERY. OPS CHECKED GOOD. AIRCRAFT RETUTNED TO SERVICE.