Hi Daox and thanks for the welcome
Yes I agree, excluding the alternator is an advantage in direct cost mpg. I guess you live in a warm climate? Im in the UK and I cant see a car battery of say 85AH lasting long if it has to take a drain of starting several times, defrosting the front screen, defrosting the rear screen, heating the cabin, headlights, radio, wipers and so on.
Lets not forget that the cost saved in mpg is offset by the cost of manually charging also.
Also modern batteries are damaged and reduce capacity if discharged past a certain level.
In some climates I guess it can work, but in others I dought its worth even trying.