Orion

The other day I'm stuck in traffic...out of no where my car all of a sudden shuts off. I also see a battery light indicator on my dash. I turn the ignition and the car starts up with no problem. Hasn't done it since yesterday. Does anyone know why it did this? I noticed my car idles at approximately 650-700 at a stop,but i dont experience any vibration like others members post. Is this idle correct? I just recently replaced the battery and spark plugs..recently had it ran through a computer and got some error for a heating system thing..not sure thats what the mechanic told me..he said it was nothing..any ideas?