AN MD80 IN CRUISE AT FL310 DECLARED AN EMER AND DIVERTED DUE TO THE BATTERY CHARGER FAIL LIGHT ILLUMINATED AND UNUSUAL BANG OR NOISE UNDER THE GALLEY FLOOR.

Narrative

APPROX 58 MINS AFTER TKOF THE BATTERY CHARGER LIGHT ILLUMINATED. THE ABNORMAL PROC WAS ACCOMPLISHED. HOWEVER; THE LIGHT WOULD NOT EXTINGUISH. THE CHKLIST REQUIRED ME TO LAND AT THE NEAREST SUITABLE ARPT AND CONTACT DISPATCH. I COORDINATED WITH ZZZ DISPATCH AND DIVERTED FLT TO TPA. WHILE INFORMING THE #1 FLT ATTENDANT OF THE PROB; HE TOLD ME HE HAD HEARD A LOUD BANG COMING FROM THE FLOOR BELOW THE FIRST CLASS GALLEY. REALIZING THE SEVERITY OF OUR ELECTRICAL PROB AND NOT HAVING ANY KNOWLEDGE OF WHAT CAUSED THE LOUD BANG; I DECLARED AN EMER. THE BANG WAS HEARD TWICE MORE BY THE FO AND THE #1 FLT ATTENDANT. FLT LANDED UNEVENTFULLY AT TPA AND WE WERE VISUALLY CHKED BY THE TPA ARRF PERSONNEL. ACFT WAS TAKEN OTS AND FLT WAS DELAYED UNTIL NEW EQUIP ARRIVED. WE CONTINUED TO DFW ARRIVING AT XA10 LCL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE CAUSE OF THE BATTERY FAIL LIGHT TO ILLUMINATE WAS THE CHARGER COOLING FAN HAD FAILED AND THE CHARGER THERMAL OVERHEAT PROTECTION SHUT IT DOWN. THE RPTR SAID AS FOR THE 3 BANGS OR THUMPS THESE WERE CAUSED BY A 'ROUND' RELAY BEING ACTIVATED. THE RPTR STATED THE MAINT PEOPLE CALL IT A 'ROUND' RELAY BUT NEVER EXPLAINED ITS FUNCTION AND WHY ITS CALLED 'ROUND.'