I guess you have their charge controler ? and are you connecting the battries in Series
for 48v or Parallel for 12v ?
I have the same gen 48v version and the 3 phases out are just a/c and in the box is diodes if you replace their charge controller (if you want to charge your battries in parallel ) with a 12v charge controler it should work because that is what determines when it should start charging the battries , not the gen it self
mine just came with the box and diodes , which is fine for me because I make my own charge controllers