Narrative:

Normal operations on flight from ZZZ to den full load of passenger and observer crew; VFR conditions. At approximately 15000 ft; both dc fail lights came on. Captain was PF; first officer got out book and we went through procedure. Battery voltage was low so we reset the circuit breaker called for in the checklist. It did not fix problem. We leveled off at 23000 ft; first officer became PF and I initiated contact with dispatch and maintenance. Our procedures call for landing at nearest suitable alternate; maintenance confirmed there was nothing else we could do and dispatch concurred with ZZZ as nearest suitable and he notified ATC of our change in destination back to ZZZ. Flight attendants were informed and given a cabin advisory with no need to prep cabin. Passenger were informed by me of nature of emergency and requirement to return to ZZZ. Aircraft was not overweight (109000 pounds.) uneventful visual/ILS approach and landing was accomplished. After landing it was ascertained by contract maintenance that we had a bad battery charger and this also led to a very overheated battery. Battery was too hot to touch for him to remove from aircraft until it cooled. Other thing to note is thrust reversers did not deploy on landing. This was not on our checklist of things that we might lose. Maybe this was caused by a related electrical problem due to the low and overheated battery; but it would have been nice to know. We did choose to land on longest runway available of 9500 ft; so it was not a problem. Callback conversation with reporter revealed the following information: the reporter stated when the inertial reference battery fail lights came on the battery voltage was low and as they went through the checklist it was no help. Procedures called for landing at nearest suitable airport. Dispatch and maintenance were advised of problem and the diversion was accomplished with no problem. Maintenance determined the battery charger had failed and damaged the battery and both components were replaced. Reporter indicated one interesting note; that on landing the thrust reversers failed to deploy and it was not noted in the quick reference handbook but should be added as what is lost when the main ship's battery and charger fails.

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Original NASA ASRS Text

Title: A B737-500 ON CLB AT FL230 DECLARED AN EMER AND RETURNED TO DEP ARPT DUE TO ILLUMINATION OF BOTH DC FAIL LIGHTS. QRH PROCS DID NOT CORRECT PROB AND DID NOT STATE THRUST REVERSERS WOULD BE INOP FOR LNDG.

Narrative: NORMAL OPS ON FLT FROM ZZZ TO DEN FULL LOAD OF PAX AND OBSERVER CREW; VFR CONDITIONS. AT APPROX 15000 FT; BOTH DC FAIL LIGHTS CAME ON. CAPT WAS PF; FO GOT OUT BOOK AND WE WENT THROUGH PROC. BATTERY VOLTAGE WAS LOW SO WE RESET THE CB CALLED FOR IN THE CHKLIST. IT DID NOT FIX PROB. WE LEVELED OFF AT 23000 FT; FO BECAME PF AND I INITIATED CONTACT WITH DISPATCH AND MAINT. OUR PROCS CALL FOR LNDG AT NEAREST SUITABLE ALTERNATE; MAINT CONFIRMED THERE WAS NOTHING ELSE WE COULD DO AND DISPATCH CONCURRED WITH ZZZ AS NEAREST SUITABLE AND HE NOTIFIED ATC OF OUR CHANGE IN DEST BACK TO ZZZ. FLT ATTENDANTS WERE INFORMED AND GIVEN A CABIN ADVISORY WITH NO NEED TO PREP CABIN. PAX WERE INFORMED BY ME OF NATURE OF EMER AND REQUIREMENT TO RETURN TO ZZZ. ACFT WAS NOT OVERWEIGHT (109000 LBS.) UNEVENTFUL VISUAL/ILS APCH AND LNDG WAS ACCOMPLISHED. AFTER LNDG IT WAS ASCERTAINED BY CONTRACT MAINT THAT WE HAD A BAD BATTERY CHARGER AND THIS ALSO LED TO A VERY OVERHEATED BATTERY. BATTERY WAS TOO HOT TO TOUCH FOR HIM TO REMOVE FROM ACFT UNTIL IT COOLED. OTHER THING TO NOTE IS THRUST REVERSERS DID NOT DEPLOY ON LNDG. THIS WAS NOT ON OUR CHKLIST OF THINGS THAT WE MIGHT LOSE. MAYBE THIS WAS CAUSED BY A RELATED ELECTRICAL PROB DUE TO THE LOW AND OVERHEATED BATTERY; BUT IT WOULD HAVE BEEN NICE TO KNOW. WE DID CHOOSE TO LAND ON LONGEST RWY AVAILABLE OF 9500 FT; SO IT WAS NOT A PROB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED WHEN THE INERTIAL REFERENCE BATTERY FAIL LIGHTS CAME ON THE BATTERY VOLTAGE WAS LOW AND AS THEY WENT THROUGH THE CHKLIST IT WAS NO HELP. PROCS CALLED FOR LNDG AT NEAREST SUITABLE ARPT. DISPATCH AND MAINT WERE ADVISED OF PROB AND THE DIVERSION WAS ACCOMPLISHED WITH NO PROB. MAINT DETERMINED THE BATTERY CHARGER HAD FAILED AND DAMAGED THE BATTERY AND BOTH COMPONENTS WERE REPLACED. RPTR INDICATED ONE INTERESTING NOTE; THAT ON LNDG THE THRUST REVERSERS FAILED TO DEPLOY AND IT WAS NOT NOTED IN THE QUICK REFERENCE HANDBOOK BUT SHOULD BE ADDED AS WHAT IS LOST WHEN THE MAIN SHIP'S BATTERY AND CHARGER FAILS.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.