[MetroMPG]

I think there's merit to the idea.

Ideally you'd want to support the system voltage at alternator levels (~13.4-14v) while running without the alternator. But of course that complicates things and raises the cost.

If saving money is your goal (vs. tinkering for fun & outright efficiency) you'd have to do the math on this. I'd be surprised if the cost of the battery (consumable too) will offset the savings in fuel.