The instruments generally run off the battery at all times. The alternator charges the battery. The engine spark runs directly off the alternator though, at least in the cars I've run with a dead battery (a couple Jeeps, a Lexus, and a Honda). If it doesn't run directly off the alternator, then maybe those batteries had just enough charging ability to act as a capacitor for the spark plugs. The car should keep running with a dead cell, but your instruments may act wierd.
Last edited by WDE46; 10-09-2012 at 11:34 AM.