A B737-300 FLT CREW ENCOUNTERED ELECTRICAL ANOMALIES DURING PREFLT AND ERRONEOUSLY ASSUMED THE PROBLEMS WERE RELATED TO LOW BATTERY VOLTAGE.

Narrative

I TERMINATED ACFT NUMBER AND TURNED IT OVER TO THE RAMP SUPVR TO BE REPOSITIONED OFF THE GATE FOR THE NIGHT. NEXT DAY; WE ORIGINATED ACFT NUMBER IN ZZZ. WHEN PERFORMING THE APU FAULT/INOP TEST ON THE BATTERY; THE FAULT LIGHT ILLUMINATED BUT THE APU DETENT INOP LIGHT DID NOT. ALSO; DURING THE SQUIB TEST; THE APU SQUIB ILLUMINATED BUT THE ENG SQUIBS DID NOT. FO NOTICED THAT THE POS LIGHTS HAD BEEN LEFT IN THE 'ON BATTERY' POS OVERNIGHT. THE BATTERY VOLTAGE WAS 23 VOLTS. I ASSUMED THE BATTERY VOLTAGE WAS JUST LOW ENOUGH TO GIVE ERRONEOUS TEST INDICATIONS. BOTH SYS TESTED NORMAL WITH GND PWR ON THE ACFT. DURING OUR 2 HR FLT; WE WERE STILL DISCUSSING THIS TEST. EVEN THOUGH WE WERE GETTING CORRECT INDICATIONS; THERE WAS STILL A QUESTION ABOUT THE INITIAL TEST. WE ELECTED TO HAVE MAINT MEET US. AFTER TROUBLESHOOTING AND CHANGING SOME COMPONENTS; IT WAS DETERMINED THAT THE ANOMALY WAS DUE TO A BROKEN CIRCUIT BREAKER. IN CONCLUSION; BECAUSE THE SYS TESTED NORMAL WITH AC PWR; I BELIEVE THE SYS WAS OPERATING NORMALLY AND SAFETY WAS NEVER COMPROMISED. SINCE THERE WAS NO OBVIOUS CONNECTION BTWN THE 2 ERRONEOUS TESTS; I ASSUMED THE COMMON DENOMINATOR WAS LOW BATTERY VOLTAGE. NEXT TIME; IF THERE IS A QUESTION; I WILL INVOLVE MAINT CTL AND NOT MAKE ASSUMPTIONS.