FLC OF AN MLG LOSES A GENERATOR CONSTANT SPD DRIVE CAUSING FAILURE OF 1 GENERATOR AND WAS UNABLE TO START THE APU FOR BACK UP PWR DUE TO A BATTERY THERMAL RUNAWAY.

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ENRTE PHL-GSO; JUST PRIOR TO TOP OF DSCNT; #2 CONSTANT SPD DRIVE LOW OIL PRESSURE LIGHT CAME ON. DISCONNECTED #2 CONSTANT SPD DRIVE. APU WOULD NOT START. ADVISED ATC OF 1 GENERATOR OP. ON APCH; AFTER LOWERING GEAR OBSERVED NO GREEN GEAR LIGHTS. MISSED APCH AND REQUESTED VISUAL ON GEAR DOWN BEFORE LNDG. TWR VERIFIED GEAR DOWN AND I REQUESTED EMER EQUIP UPON LNDG. LANDED AND TAXIED TO GATE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THE APU WOULD NOT START BECAUSE OF LACK OF SUFFICIENT DC BATTERY CURRENT TO START THE TURBINE. HE FOUND OUT AFTER ACFT MAINT EXAMINATION; THAT THE BATTERY HAD A 'THERMAL RUNAWAY' APPARENTLY DUE TO A FAULTY REGULATOR. OF COURSE WHAT STARTED THE CHAIN REACTION WAS PROBABLY THE GENERATOR CONSTANT SPD DRIVE FAILURE AND THE RESULTANT SWITCH OVER OF ALL ELECTRICAL PWR ON THE 1 GENERATOR RATHER THAN THE SUPPLEMENTATION OF THE APU ELECTRICAL GENERATOR INPUT. THIS PROB CAUSED THE GEAR LIGHT OUTAGE AND OTHER ELECTRICAL OPERATED INDICATIONS AND SYS. HE FURTHER STATED THAT WHEN HE BEGAN TO SMELL SMOKE FUMES DURING THE LNDG INDICATION LOW FLY BY; HE BECAME WORRIED AND DECLARED AN EMER FOR AN IMMEDIATE LNDG. HE FOUND LATER THAT THE SMELL CAME FROM THE 'RUN AWAY' BATTERY. HE ALSO STATED THAT THE FAA SENT HIM A LETTER OF INVESTIGATION FOR POSSIBLE VIOLATION OF THE FAR FOR NOT WRITING UP THE FACT THAT THE APU WOULD NOT START! HE IS IN THE PROCESS OF REPLYING TO THAT LETTER.