What was found
APU WILL NOT START. ACFT GROUNDED. R & R STARTER GENERATOR IAW AMM 49-41-21-000-801 & 49-41-21-400-801. STARTER OPS CHECKED GOOD ON GROUND POWER. AFTER SWITCHING OFF APU GENERATORS, APU AUTO SHUTDOWN. PERFORMED APU BITE PROCEDURE 49-60-00-810-801. FOUND FAULT CODE 49-11019 DC POWER LOSS SHUTDOWN POWER INTERUPT. SUSPECT SPCU BAD. REPLACED SPCU REF AMM 24-34-11-000-801 & 24-34-11-400-801. SPCU CHECKED OK BUT DC POWER LOSS SHUTDOWN REMAINS. FOUND MAIN BATTERY VOLTAGE OK BUT UNABLE TO KEEP LOAD. R & R MAIN BATTERY, OPS CHECKED OK IAW AMM 24-31-11-000-802-002 & 24-31-11-400-802-002. PERFORMED APU OPS TEST, CHECKED GOOD REF AMM 49-11-00-710-801. NO FURTHER DEFECTS NOTED. REPLACED AUX BATTERY IAW AMM 24-31-11-000-802-002 AND 24-31-11-400-802-002. OPS CHECKED GOOD.