As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOT REPORTED SEWAGE/ SULFUR SMELL. VOLTAGE ALSO DROPS TO 13 VOLTS ON START UP AND TAKES A WHILE TO RECOVER. REMOVED AND REPLACED BATTERY PERFORMED GROUND RUN AND 10 MIN MAINTENANCE FLIGHT. NO DEFECTS NOTED IN REF TO BELL 407 MM CH 24.