LTT FLC FLIES TRIP WITH NO TAIL BATTERY INSTALLED. NOTICES ON SHUTDOWN WHEN VOLTAGE GAUGE GOES TO 0.

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AFTER ALL CHKLISTS COMPLETED AS PER SOP; FLT DEPARTED FOR BWI. AFTER PARKING THE ACFT IN BWI; UPON SHUTDOWN; CAPT NOTICED A DISCREPANCY IN TAIL BATTERY VOLTAGE (0 VOLTS). CAPT CONTACTED MAINT; MAINT REPAIRED THE ACFT AND ADVISED THE FLC THAT THERE WAS 'NO' TAIL BATTERY IN ACFT. UNDER NORMAL CONDITIONS OF FLT; THIS WOULD NOT BE NOTICEABLE. THE BATTERY WAS INSTALLED AND THE ACFT CONTINUED THE DAY WITH NO FURTHER INCIDENT. WITH EXTERNAL PWR CONNECTED TO THE ACFT; THE TAIL BATTERY VOLTAGE WILL READ 'SYS VOLTAGE.' IT IS POSSIBLE THAT DURING THE PREFLT CHKLISTS IN MDT; THIS 0 VOLT DISCREPANCY WAS UNNOTICED BECAUSE THE ACFT WAS PWRED EXTERNALLY. THE TAIL BATTERY SERVES A SECONDARY FUNCTION ON THE ACFT; THEREFORE; NO NOTICEABLE DISCREPANCY; OTHER THAN '0 VOLTS' INDICATED ON INTERNAL PWR ONLY; WOULD BE APPARENT UNDER NORMAL CONDITIONS -- TKOF; CRUISE AND LNDG. THIS SIT CAUSED NO INCIDENT AND NO PROBS HAVE OCCURRED BECAUSE OF IT. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: RPTR STATES THE FUNCTION OF TAIL BATTERY IS TO ACT AS A SECONDARY CHARGE FOR THE IGNITERS. THE ENGS; HOWEVER; WILL START WITHOUT IT. PREFLT DOES NOT INCLUDE A TAIL BATTERY CHK AND THERE IS NO SPECIFIC CHK FOR BATTERY ON ACCEPTANCE. ON INTERNAL PWR THERE ARE 3 SEPARATE VOLTAGE INDICATIONS BUT ON EXTERNAL PWR THERE IS ONLY AN INDICATION OF THE TOTAL SYS VOLTAGE. THIS ACFT WAS ON EXTERNAL PWR. THE CHIEF OF MAINT SAID NO CREW PROB WOULD OCCUR AS THIS WAS COMPLETELY A MAINT OVERSIGHT. SOME TIME AFTER THE EVENT; THE CAPT WAS CALLED IN FOR REVIEW OF THE SIT AND THE FLC BECAME VERY CONCERNED; THAT IS REASON FOR RATHER LATE FILING OF RPT. RPTR FEELS PROC SHOULD BE ESTABLISHED TO GO TO INTERNAL PWR AND THEN CHK VOLTAGE. FO UNABLE TO SEE GAUGES FROM HIS POS.