My 54 3100 would not charge an 8v battery. At all. However, we ran 6v tractors back in the day and charged 8v batteries with 6v generators by removing the VR and replacing with a simple cut-out relay. Still works on my JD B.
Impractical if not impossible. Another However though... one cannot have both 6v and 12v batteries on the same charging system.
My solution to a similar problem back in the 80's was a 12v battery in the toolbox to run some 12v accessories... a CB radio and some tunes. Once in a while on the trickle charger kept everything happy.
