Sorry for being such a blabber mouth here, but you've got me going. Actually, you've provided me a good way to avoid doing that SG check on my batteries.......
I've had two cell failures on my super expensive, super uber all that Surrette batteries. About 2 years into service I started getting low voltage alarms. SG was fine on all cells, but I ultimately tracked down one battery (each battery is two cells, so 4V) where the voltage would be fine, then plummet during discharge. One nice thing about the Surrettes is that you can pull off the cover (there is an outer case with the actual cells inside) and access the individual cells which are bolted together. I was able to further isolate the problem to a single cell. Again, its SG was fine, but as it would discharge the voltage would plummet and actually reverse voltage. After a huge run-around from Surrette, they finally agreed to replace it under warranty, but..... I would have to pay shipping at a cost of about $200, and it would take 2-3 months to get a replacement cell. WTF?
So I reconfigured the bank to run at 44V instead of 48V, adjusting charge parameters accordingly, and kept running until the replacement cell arrived. BTW, this is an other advantage to having individual cells accessible. The new cell ultimately arrived and with the help of a couple of friends, we were able to get it replaced. BTW, one of these batteries weights 325lb, and an individual cell half that, so handling them is not easy.
But the new cell always ran with very high SG. 1325 to be specific. I asked Surrette about it and they just hand waved it away and said it would be fine. Well, about a year ago I had another cell crap out, and guess which one it was? That's right, the same darn cell.
This time I got a little smarter and called around, ultimately finding a dealer who would sell be a replacement cell for $300 including shipping. This was way more attractive than dealing with Surrette on a pro-rated warranty claim, so I ordered the battery.
As an aside, at the time I considered dumping the whole bank and going to AGMs, but I couldn't find anyone who had them in stock in L16 size, so I abandoned that approach. And besides, if for $300 I could get another couple of years out of the Surrettes, it might give me time to switch to LiFePO4...
This time the replacement battery took nearly 4 months to get. But it's installed and I'm up and running at full voltage again. Just need to go do another check on SG, water level, etc. to be sure all continues to be well.