it's not free, just like a heavy jacket is not "light"

you put it on, think, yeah, this is nothing extra, but then after you have been in it half a day you realise you are becoming fatigued a lot faster

extra load on the battery is going to mean extrta load on the alternator which is going to mean extra load on the engine which is going to mean more fuel is spent

the only time it's free energy is when your destination is at a lower altitude and your engine is idling and the alternator is catching power from the spinning of the wheels, and i'm not even sure that actually happens, i am pretty sure alternators are directly coupled to the engine