As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
VISIBLE SMOKE AND ELECTRIC SMELL IN CABIN AROUND ROW 42 ON GROUND, DURING BOARDING. FOUND APU BATTERY HOT WITH SOME SMELL COMING FROM COOLING VENTS. REPLACED APU BATTERY AND CHARGER PER MM CIRCULATED AIR THROUGH CABIN WITH PACKS. NO SMELL OR SMOKE.