Charging problems!!!! stumped!

Ok heres the deal. I have a 96 3500 dually 12 valve. I have been having charging problems for quite some time now and has only gotten worse and now I am completly stumped. A while ago the truck was not charging so I put a new alternator in but that wasnt the problem and later found out that the pcm went bad(internal voltage regulator) so I went on the forum and found out that most people just go to the parts store and buy an external regulator and wire it in so thats what I did. Everything seemed to work fine for a while but is now frying batterys and charging at 15.9 volts. Followed all the grounds and cleaned everything and made sure all of that was fine, put a new alt on, multiple regulators, and new batterys multiple times now because its frying and bulging the batterys now. i am completly stumped and do not know what to do any more. Is 15.9 volts to high? is the regulator bad or just bad quality? I have no clue what to do anymore ao any help would be very much appriciated! Thanks in advance, Nick.