What was found
APU WILL NOT START FAULT LIGHT ILLUMINATED ON APU PANEL. ACFT GROUNDED. FOUND FAULT LIGHT ELEC PERFORMED BITE TEST IAW FIM 24-31, TASK 801. FOUND STATIC INVERTER INOPERATIVE. PERFORMED FIM TASK 24-34, TASK 801. STATIC INVERTER FOUND TO HAVE LOW AC VOLTS ON TEST. REPLACED FWD MAIN ACFT BATTERY IAW MM 24-31-11-400-802-002. REPLACED AUX BATTERY IAW MM 24-31-11-400-802-002. BOTH OPS CHECKED NORMAL IAW MM 24-31-11-400-802-002. RESET ELECTRICAL FAULT REF FIM 24-31, TASK 801. APU STARTS NORMAL REF MM 49-11-00-860-801.