Volvo Repair: Electrical Problem, volvo dealership, dead batteries

Question
My wife has a 2002 S60.  The other day while driving home she got an electrical system issue message on her display screen so she headed towards the Volvo dealership.  On the way, the idiot lights started to come on one at a time then the radio went dead.  Then it started to act sluggish before it finally stalled out a mile from the dealership.  The electricity was gone.

The mechanics at the dealership  say it was a dead battery.  I've had many dead batteries in my life in many different cars but never have I seen a car actually run out of juice while driving.  Are they correct or could it be something else like the alternator?  

Answer
if it was by driving the battery  is not supplying so much power little comes from  batter when you drive  the most power for you electrical system comes from alternator when you drive  it was most likely not charging alternator  for  little time and than started to work  .you can wait see  if it happens again or go with alternator now if you from chicago i can help  .good luck