You are not being clear about that 12v. The voltage during the test is the loaded voltage. That bounces back after the test, and that voltage is the one that tells you your SOC--but only after time has passed so you are closer back up to a "resting" value.
Eg with Wet batts, where 12.2 is 50%, they will be at 11.6v under a 20 hr rate load at 50%, and will bounce back to 12.2 by next day. AGMs have higher voltages per SOC, but not all AGMs are the same for that either. Mine are about 12.4 at 50% and 13.0 full.
You need a way to tell what your batteries' SOCs are, to do a capacity test. An AH counter is the easiest way. You can use SG with Flooded but not with your AGMs. Voltage per SOC does not work unless you have a good voltage per SOC table for your type of battery in their spec sheets, and it is a next day thing to take the measurement.
IMO you cannot do a capacity test while popping by the storage yard for a few hours to run a test. You can do a test to see if the batteries are any good at all, but that might be all you want to know.
If you put a load on them and their voltage does not "collapse" but goes down in a regular way, you are good to go. Some time when you have the measuring equipment and the time, and shore power, you can do a proper capacity test. I do it with my Trimetric for AH counting, and only run from full down to 50% or 10 hours, whichever comes first. Then just double the AH. No need to do the 20 hours all the way down.
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