As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
LOW BATTERY POWER MSG. APU WILL NOT START. A/C CONNECTS WITH GROUND POWER. GROUNDED - YES. REPLACED START POWER UNIT PER B737-900 FIM 49-40 TASK 822 STEP F. (5) & B737-900 AMM 49-41-71. PERFORMED TWO SUCCESSFUL BATTERY STARTS. MAIN BATTERY VOLTAGE NEVER DROPPED BELOW 20 VOLTS DURING START.