I have replaced the alternator and it still is doing the same thing. One battery is charing at 12 the other is at 19 and overcharging obviously.

 

It is a 2003 dodge 3500.

 

Is it a service truck; does it have aftermarket devices spliced into the overcharging battery?

both batteries have been isolated and tested individually? They cannot be tested properly without isolation.

 

Thanks so much for replying. It is not a service truck in the sense that it does not have a built in air compressor. It has an aftermarket pump that has some corrosion on the connection. The connection that the battery is charging at 12, and the side with no aftermarket stuff is at 19. I think there is a short in there causing one battery to over charge. One place 4 or 5 months ago said it was the alternator and that seemed to have fixed things for a few months. Now I am having the exact same issue again where it is overcharging a battery, the passenger side battery if that matters. I am going to be redoing each battery terminal connectors along with the aftermarket fuel pump. I think this will probably be the issue, something a local shop should have been able to point out with a first glance at it. I did have both batteries disconnected and one is high and one is a little low. The high battery getting charged is a brand new battery as well so it is not a battery issue.

 

You should really replace both batteries as a pair. If the driver's side battery is only charging at 12v, it sounds like it's not charging. I'd start doing voltage drop tests on your battery cables. The driver's side battery has a temp sensor under it to prevent overcharging. If you have excess resistance and that battery isn't charging enough, the temp sensor isn't going to tell the ECM to cut back on the charging, so the alternator is going to go all in, burning up the battery on the passenger's side. Check the battery cable between the B+ on both batteries, and the ground cable for the driver's side battery. A simple continuity test isn't going to cut it, because if you've got one single strand between the ends, you'll see good continuity. A voltage drop test will tell you where you're losing voltage.

Have you swapped batteries side to side?