If they've gone bad, it could take a lot of work to restore them.
If they're still good, you could be throwing away something that still has useful life left in it.
You won't know until you do a test.
Leave them on a charger and keep 'em at float for a few days or a week. Let them rest a day. Connect a load that will draw about 1/20th of their capacity in amps.
For example, if the batteries (together) have a rated capacity of 200 Amp-hours, then 1/20 of 200 Ah is 10 Amps. A few auto headlights will do. Connect them and leave them turned on for a long period of time, which will drain the AGM's.
You won't know anything if you don't monitor the voltage and current through the whole proces. Float voltage, resting voltage, battery voltage and current when you start the discharge test (and the time), battery voltage and current 1 hour into the discharge test, 2 hours, and so on, until the battery has been run down to, say, 11.5 volts. Recharge immediately.
If it takes 10 hours to discharge 10 amps then you have 100 amp-hours. Typical battery AH ratings are for discharges down to 10.5 volts (I think) so by stopping at 11.5V you are taking out only what would normally should be taken out of a lead-acid battery for it to cycle often and not be damaged. The trip from float charge down to this level of discharge is about 75% capacity (I can look up the exact number if you need it), so that's enough information to figure out what the battery is worth without discharging any lower.
It's best to start a test like that first thing in the morning, on a day you're not going away for a long time, check often, and maybe the test will be done by supper time. If the batteries are done before lunch, they're no good.
