I clear the code it comes right back. When I test batteries with truck off I get 11.80V.
Engine running with no electronics on = 12.80 - 13.2V
Engine running with a ton of stuff on (AC, headlights, radio) = 12.3V

Revving gets those numbers up somewhat. But it never really maintains a reading above 14V. If I have absolutely no electronics on and really step on it while driving I can briefly get a 14.2 reading. But as soon as I come off the pedal, she's back to the mid 12's.

I cleaned all 4 connections and battery terminals along with a coat of copper based anti-seize on the terminals. I also inspected wire hardness to Alternator. Looks good.

What do I check next before throwing in a new Alternator?

Few things to note. Both Batteries less than a year old. Interstate Battery Brand bought from Costco. Single Alternator - Probably original.

 

A fully charged 12v battery with nothing hooked up should read roughly 12.6 volts...so yours is reading a tad low...however it's prob not the batteries fault......
With the engine running and alternator charging at idle you should get somewhere between 13.7v and 14.7v give or take......With the readings you stated above it looks like your alternator is bad and not producing enough voltage and needs to be replaced.
Now the question is has the faulty alternator ruined the new batteries? It's possible...I'd have them load tested just to be sure and so you won't ruin new alternator.

 

Is the best way to do this is to fully charge them on the bench with a trickle charger and then bring them to Autozone, etc for the load test? I wouldn't mind spending a few bucks on a home load tester. Any recommendations on a decent one that can handle the amps that these batteries put out? Or are they very expensive?

 

Sorry for the delay.....yes your best bet is to take them to someplace like that and have them tested....Charging them up first isn't a bad idea.
Not sure on home load testers.

 

sounds like the batteries are old and need replaced to me.


Verify the connections at the alternator are ok.

 

I installed a new MotorCraft 200 Amp Alternator. This seems to have done the trick. I was going to replace the batteries too, but after installing new alternator and driving around awhile the batteries have fully charged and have passed a load test.

Here's a video I took of the steps I performed to change out the alternator.