As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING START THE BATTERY HOT CAUTION LIGHT ILLUMINATED, PILOT ABORTED START AND FELT THE BATTERY. THE BATTERY WAS EXTREMELY HOT TO THE TOUCH. A REPLACEMENT BATTERY WAS INSTALLED. ACFT WAS GROUND RUN. ALL ACFT SYS SEEMED TO BE OPERATING CORRECTLY AND CHARGE VOLTAGE WAS CONFIRMED TO BE CORRECT AND STEADY. ORIGINAL BATTERY WAS SENT FOR DEEP CYCLE. THERMAL RUNAWAY IS SUSPECTED.