As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APU WON'T START, THREE ATTEMPTS. GET FAULT LIGHT EACH TIME. ACCOMPLISHED APU BITE CHECK FOUND NO ACTIVE FAULTS. STARTED APU NORMAL, OBSERVED APU BATTERY VOLTAGE DROPPED TO 16V, SHUTDOWN APU AND OBSERVERED APU BATTERY CHARGER. AMPS LESS THAN 38.5 AMPS. REPLACED APU BATTERY CHARGER PER MM, OPS CHECKED NORMAL