Tony R
possible bad battery connection ,jump it and see what the charge gauge reads ...should be 13.5 to 14.5 volts

joemong
defective batteries or battery.

iwannaknow
diesels use a lot of your battery charge it sounds like to me you need a new alternator and a better one than stock...but i would still check around with a voltmeter to make sure nothing was grounding out that shouldnt be or you have any melted cracked or broken wires

Macs-batteries
All batteries will be tested dead if your alternates not charging batteries have to have a full charge to get a true reading so i would look at the alternator

CLAYTON M
charge the battery's take the cables off - make sure its cold put cables on wait 15 minutes - do not start - put your hand on the starter our checking for heat if warm it has a short in it AAA battery trucks can do a drain test - but they can't tell were it at but how fast the power lose is

Matthew B
Go to autozone have them test the alternator.

Al K
When I was working as a mechanic we used to find that probably 50% or more of the batteries that were replaced by the owner were fine and that the scenario you described happened to them. So first I would check the cables to make sure they are clean and tight. This includes the connections to the alternator, I would imagine the regulator is internal on the alternator but I am not sure. Then I would check the alternator output. The problem is that they voltage may be OK but the amperage may be too low from the alternator. The best way to do this is to have them check it in the car with the engine running under load and no load. First with just the engine running then with all the accessories like lights, A/C radio, etc that load the alternator. My bet is your alternator is not doing so well. It;s a shame you had to trash two good batteries but if they were older ones it might be just as well. I have seen slipping belts be an issue too, not sure your setup is on your truck. But you don't want to replace the alternator and still have a problem so I would double check all the simple stuff first. If the alternator is easy to get off take it in and have it bench tested which will nail that down yes or no.