I have a smart alternator. I found that as a hypermiler, it was struggling to keep my battery charged - in my city driving there aren't enough revs to really charge up the battery. I have a 1.6l 2.1 ton diesel so my in gear glide distances are much better than a throttled engine in a Mirage.
I added solar panels.
I did the maths on the solar panels 360W saves a theoretical 0.02l of fuel per day - more or less independent of distance driven.
Sometimes it pays to just know when to quit
