charging W900 6 volt batteries

Can't for the life of me figure out how to get the batteries on my 1976 W900 charged using a 12volt charger.

system has 4 6volt batteries and two isolator switches, I know I have successfully charged the batteries using a 12v charger in the past, just don't remember how I did it.

people have suggested just going to two 12volt batteries in the past, but I had to get a new 12-24v series parallel switch for it and when I did the guy at the electrical rebuilder shop said I was better off with...

charging W900 6 volt batteries

charging W900 6 volt batteries