Ford 6.0L Power Stroke Diesel :: Batteries Get Discharged To A Certain Point / Alternator Not Charging

I ve been noticing that the alternator isn't charging correctly, or at least it seems like it. We drive short trips sometimes two times a day, a mile or less. One day, night actually, I noticed it started to not turn over with it's usual enthusiastic sound. When the batteries get discharged to a certain point, the voltage at idle will be less than 12 volts. If I rev the engine for a bit, the volts jump up, but at idle it drops back to 12 or below. Seems to me that it should be at least 12v or above if the batteries are discharged, and more towards 12.5ishV if the batteries are fully charged.

After I put it on a 6A charger overnight, the voltage upon starting is above 12, sometimes over 13. But if the batteries start to dwindle down from short trips, the voltage drops to 12 or below at idle.

I did install the smaller pulley a few years back, and that worked massively. I couldn't get 12V at idle before installing the smaller pulley. But now it seems like I'm back to that same situation before installing the smaller pulley.

So, I m guessing that a alternator bench test is in my future? The batteries are fairly new, tho they have been down to nothing at least twice from boneheaded moves by yours truely!

And, I'm speculating that this charging situation contributed to the recent FICM replacement that I went thru.