GMC Repair: electrical problem, warranty replacement, regulator problem

Question
I have a 6.2 diesel suburban, brand new alternator, 2 brand new batteries and they will not charge up.  Battery condition shows apx 8 volts after I drive it a while -- alternator was just installed;  the removed one was tested & worked fine but got a warranty replacement from Napa anyway.  I would think it would be a regulator problem if this was a generator, but I understand there are no regulators with alternator equipped engines.  Any ideas?  Sorry for asking about an older vehicle--I figured electrical problems are pretty much universal.  Thank you!

Answer
Hi
What is the rate of charge off the back of alternator? If it is proper 13.5-14.5V then you have a wire issue from the alternator to the battery(s). If you only getting battery voltage at the back of alternator, make sure the small wire to alternator is getting battery voltage also. If not it may be a fuse or another wire problem. The alternator has to have current to make current.
I hope this helps
Good Luck
God Bless
Pawl