Today I revived two more AGM batteries at work, so I have three in total. Not sure if I will be able to keep them or not, but here's hoping. All three had been discharged to the trucks's 10.5V cutoff and left that way.
All I have at work is a dumb charger. It has three settings: low, medium and high. All three seem to be constant amperage. I charged and load tested each battery individually to get an idea of their health. I have never charged AGMs before and I found it interesting how they acted. At first they wouldn't eat current. They sat at 14.0V and took about 8A, which I thought meant murdered batteries. After about 10 minutes, the voltage suddenly fell to 12.5V and the current went up to 15A, as if they suddenly woke up. I jacked the charger up to the high setting which meant 13.8V and 50A for about 2 hours. I monitored the temp with an IR gun and they only got to about 80F, well below the danger zone. As the voltage finally climbed to 14.5V, I lowered the charging speed until finally on low they were at 14.5V, drawing 5A. Again, I did this to each battery individually.
I tortured them all on the carbon pile with 350A for 15 seconds and none sagged below 10V.
I also found a good solution for my "10-pin Charging Connector" in the drawing above, if I end up going that way. It actually has 12 pins in case I need more. Note the fancy Caterpillar yellow paint. It's bits from a Cat 15L engine harness.
It even has mounting holes!