I just got finished tearing it down and rebuilding it; and indeed TWO golf cart batteries in one quadrant (it's arranged as 4 batteries in parallel (one quadrant) with two quadrants in series; giving 12 volts; then two halves in series giving 24) were really shot, putting out 3 volts at no load. The other batteries in that quadrant were only putting out 4.5 by now; but probably due to massive over loading; hopefully they'll recover.
Those symptoms mean some battery or cell in the arrangement is dead; and pulling everybody else in the system down with them. About 90% of the time. The other answers folks came up with could yield very similar symptoms- but are going to be much rarer occurrences.
One of the esoteric facts that it helps to know- a "dead" lead acid cell will usually read full high voltage- immediately when a charging current is applied. But- in fact it is not accepting the charge; it's dead. So when a load (greater than the charging current) is applied- it will very quickly drop down to a bad number- like 22V.
When I left the battery bank; after 45 minutes of 28 amps charging from the diesel generator; to try to bring some of the drastically low batteries a little back in line; the computer was reading 26.5 VDC; under a 3 amp load (heat lamp for chicks). Which is really pretty good.
If the bank is fixed (sort of) - in the morning, I will expect it to read around 24.5 or so, before the sun hits the panels. That should mean it'll hold up for a while; though some of the batteries that were pulled way down by the dead cells may prove to be too damaged, in the long run.
The batteries that went bad- were some of the newest. And in the next quadrant; some of the oldest (back to 2001!) are still holding a steady 6.3V; even after working with dead cells in the system. My guess is- the quality had declined.
ALL experts on solar system battery banks say: "NEVER put new batteries into a bank with old batteries! Your new cells will be pulled down to the function level of your worst cells- and their life will be shortened."
Fine. True. But I think those guys are not paying for the 16 golf cart batteries themselves.
I just can't afford to replace the bank, when one battery goes south. If I did; I'd have bought about 4 sets of 16 by now. Instead of which, I've bought about 2.2 (since 1994). Which is a bloody great lot cheaper.
One thing I forgot- when the bank is under stress and out of balance; some of the batteries will wind up using a lot more water than others. I found several cells with the water out of sight below the plates. Naughty. And; my backup distilled water supply was not adequate; I eventually put almost 3 gallons of distilled water into the bank; quite a lot; and I had to go in to town to get it.
Had to; though; the chicks were without heat! If I'd thought a little further, we could have bought extra water at the same time as we got the batteries. Duh.
The revolt of the machines continues, too- my multi-meter; really needed to check the exact status of each battery- worked super until I got the two really dead batteries out, and the new ones in. And then it quit working. And I wanted to do a little more checking; of course. I'm hoping it's just a dead battery. Hm. It DID let me finish the critical work. Maybe it's actually on my side?
And. The diesel backup generator- has a new rattle. Under load; but not when idling.