What was found
STANDBY PWR OFF LIGHT, LIGHT ON WITH AC POWER SUPPLIED AND STANDB Y POWER SWITCH AT AUTO OR BAT PERFORMED A DEBRIEFING WITH FLIGHT CREW AND THE CAPTAIN REPORTED THE SECT 2 BATTERY BUS WAS OFFLINE WHICH RENDERED SEVERAL SYSTEM INOP INCLUDING FIRE DETECTION IN CARGO PITS, E2 AND APU. THE CAPTAI ALSO NOTICED THAT WHEN HE ATTEMPTED TO START THE APU THE APU GEN LIGHT FAILED TO COME ON. CONTROL, POWER, OFF 29-2499-9-0001 2634 ON 0000 CONTROL, POWER, OFF 29-2499-9-0001 2634 ON 29-2499-9-0001 8542 FOUND C/B C5 "BAT BUS SEC 2" OUT ON SPCU. CLOSED C/B, C/B STAYED CLOSED. STBY POWER OFF LIGHT EXTINGUISHED, ENG FAIL MSG CLEARED AND APU GEN LIGHT RETURNED TO NORMAL. PERFORMED GVI BEHIND P6 PANEL, NO DEFECTS NOTED, NO SIGN OF ARCING OR WIRING DAMAGE. REPLACED SPCU AS PRECAUTION. NEW SPCU OPS CHECKS GOOD. ALSO PERFORMED DUAL ENG RUN AND VERIFIED ACFT ELEC OPERATES NORMAL ON ENG POWER. NO FAULTS NOTED. REF AMM 24-34-11 AND WDM 24-34-11-1 ROTABLE PARTS CHANGE WAS INDICATED - TT # 1506683 SN OFF 2634.