Well, it’s a good idea except for something my father said once, "You can't make what you don’t have". A windmill system that you described could partially charge the battery as you drive, but you will always lose more power than you gain. Basically, what you are describing a perpetual motion type machine, one that once started could conceivably continue to run forever, this is impossibility. I once actually had a very similar idea; I took a high powered electric remote control car and attached a fairly efficient windmill type generator to the top. It did indeed extend battery life somewhat, but not by much and it certainly could never fully charge the battery.
Even IF it was a windy day thus adding to the power created by the wind created by the car, you will still never achieve the desired results. This is because you lose power when you make the conversion of "wind" energy to electrical power, through the wiring that connects the windmill to the battery, and the battery itself as it sits.
It actually is a good idea to add a windmill type generator to the car, so that while in motion, the windmill could assist the plug, motor, or whatever other device is charging the battery, thus extending battery life.
That’s all, Chucky D