Changing battery Bank of 4 batteries--AGM?

Well, how's this for timing. Last night the lights began to dim and have now gone out. Battery voltage this morning is at 6.4! I reset the Xantrex PS inverter/charger, and my gauge says it immediately started bulk charging, charging amps went up to around 88 and voltage started going up. Then after a minute it shut down, that is, stopped charging, then restarted, and followed that pattern for a while. One battery is warm and slightly swollen. I think my batteries are indeed dead! I'm not sure whether the Xantrex has some sort of safety shut off with a bad battery or not, but after resetting it did immediately start bulk charging so I'm hoping its not the problem. I replaced my old Freedom modified sine wave unit for this pure sine wave one (2000 watt) a couple of years ago.