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.