Narrative:

After departing ZZZ to ZZZZ1; just over an hour into the flight; the 'IRS dc fail' lights illuminated on all 3 IRU's and on the EICAS screen. The QRH calls for no crew action; just listing a 'condition' that there is no dc backup power. We called dispatch; who then connected us to maintenance control. They suspected the ship's battery was dead (or nearly dead). Both maintenance control; dispatch; and myself agreed that continued flight all the way to ZZZZ1 with the aircraft in this condition would not be the safest of decisions. I determined that a diversion to ZZZZ2 would be best as it is a maintenance base and only 1 hour flight from present position. Dispatch then sent me a message stating that if I felt that the situation was not 'time critical' they would like me to return to ZZZ. We were day VFR; there was no indication that we were in any life threatening situation such as a fire or overheated battery; and virtually all system; including the IRU's; were functioning normally. On that basis I determined that our situation was not 'time critical' and returned to ZZZ. An emergency was declared and an overweight landing accomplished in ZZZ without incident.

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Original NASA ASRS Text

Title: A B767'S BATTERY FAILED IN FLT CAUSING THE IRU'S TO DISPLAY 'IRS DC FAIL.' AN EMER WAS DECLARED WITH A RETURN TO LAND.

Narrative: AFTER DEPARTING ZZZ TO ZZZZ1; JUST OVER AN HR INTO THE FLT; THE 'IRS DC FAIL' LIGHTS ILLUMINATED ON ALL 3 IRU'S AND ON THE EICAS SCREEN. THE QRH CALLS FOR NO CREW ACTION; JUST LISTING A 'CONDITION' THAT THERE IS NO DC BACKUP PWR. WE CALLED DISPATCH; WHO THEN CONNECTED US TO MAINT CTL. THEY SUSPECTED THE SHIP'S BATTERY WAS DEAD (OR NEARLY DEAD). BOTH MAINT CTL; DISPATCH; AND MYSELF AGREED THAT CONTINUED FLT ALL THE WAY TO ZZZZ1 WITH THE ACFT IN THIS CONDITION WOULD NOT BE THE SAFEST OF DECISIONS. I DETERMINED THAT A DIVERSION TO ZZZZ2 WOULD BE BEST AS IT IS A MAINT BASE AND ONLY 1 HR FLT FROM PRESENT POS. DISPATCH THEN SENT ME A MESSAGE STATING THAT IF I FELT THAT THE SITUATION WAS NOT 'TIME CRITICAL' THEY WOULD LIKE ME TO RETURN TO ZZZ. WE WERE DAY VFR; THERE WAS NO INDICATION THAT WE WERE IN ANY LIFE THREATENING SITUATION SUCH AS A FIRE OR OVERHEATED BATTERY; AND VIRTUALLY ALL SYS; INCLUDING THE IRU'S; WERE FUNCTIONING NORMALLY. ON THAT BASIS I DETERMINED THAT OUR SITUATION WAS NOT 'TIME CRITICAL' AND RETURNED TO ZZZ. AN EMER WAS DECLARED AND AN OVERWT LNDG ACCOMPLISHED IN ZZZ WITHOUT INCIDENT.

Data retrieved from NASA's ASRS site as of May 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.