> The alternator usually provides all of the current needed to run the 
> accessories when the engine is on. This can be proven if you've ever had 
> to jump an entirely dead battery and drive to the local auto shop to buy a 
> replacement...

A lot modern cars already disconnect (electronically) a partially-charged 
battery while cruising to reduce fuel consumption.  Only when you lift off 
the throttle or brake will it connect it up to be charged, thus avoiding 
wasting fuel to charge the battery.

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