this one is slightly out of my experience.
A pair of 6v us2200 copies, so 232Ah. Left on a boat for 3 months this winter with 80W of solar, commoned with a start battery 664 type.
I returned to the boat to find a 2W LED light had been left on and both batteries flat* - though the light was still on. They've been on charge for a couple of weeks, being gently cycled with 3 equalisation charges.
*flat = the 664 at 10.6V the 2200s at 7.4V
The 664 has recovered fine, is now sat at 12.8V and after being left a couple of days starts the engine easily.
the other two are behaving a little strangely. they seem to charge OK, take them off and they run low power loads fine, seeming to have capacity. As soon as I put a medium load - theoretically around 10A on them they just die, going down to a terminal voltage of 4V (!)
now my feeling is they are dead. so far so simple.
but they have had an easy life up until now, 3.5 years old, landline, easy loads, constant solar so it seems a little odd they should 'just' die.
what is going on in the battery that causes the death? is it extremely heavy sulphation? have they shed their lead?
any ideas as to a revival programme or shall I just junk them?