But while we're on the subject of these batteries, here's what I've been seeing over the years, as I have watched the health of my battery bank.
Having 24 cells to measure, and rather quick process to put a voltmeter on them all and write down the voltage in each is rather easy, but I admit I don't do it very often. Somewhere along the way, I realized that I could use some statistics to better judge how much "scatter" there is between these cells.
I installed the batteries in 2007. Before I got them they had been abused, so it took some time for the scatter to come down to a rather low level. Since then the scatter has crept up slowly. Last year for some reason it jumped up, but hasn't gone up since then.
All of the values on the chart are the standard deviation at float charge. I also measure at static charge (at least 6 hours since last charged) but I think mixing the numbers would confuse the picture. This is clear enough. I also plotted the range between maximum and minimum, because there are a few bad cells. If they ever start to get really bad, they will drag the range up before they affect the standard deviation. They are the culprits for the increased scatter starting in 2017.
The standard float voltage for these AGM lead-acid battery cells is 2.25, so a 0.02V standard deviation is only 1%. Not bad for 20-year-old cells!