" In general when one 6V battery goes bad the other in series will quickly follow."Well, of course in at least one sense almost all batteries are in series, except for 1.5 volt cells. What I have seen cause issues however is 6 volt batteries in series/parallel to form large 12 volt banks. In general when one 6V battery goes bad the other in series will quickly follow. I have replaced one (really quick problems) and both in the string. In either case, in short order the battery will not charge above what the others in the array are, as measured with a hydrometer. Removing the batteries from the bank and charging them separately they still would not charge fully. Even an EQ charge won't remedy it. So they had been degraded by being in parallel with the other batteries. Remember, this happened when both batteries in series were replaced. I have seen the same thing when 12V batteries were used to form the bank.
I very much agree that inserting a new battery in series with old ones is a quick recipe for trouble. But parallel use will too, just not as drastically as series. I have seen this on multiple occasions back in the day when we all ran large banks of 6V FLAs to form large banks of 12V. Back then 12V inverters were all that was available and the banks were very large parallel arrays. I admit the data is fuzzy because there are other factors at play. Asymmetrical connections, unequal cable lengths, some connections not as good as others are all influencing pack aging. Nonetheless, I have seen batteries sink to the lowest common denominator within a couple of months and be unable to charge fully. Even within a couple of weeks they will read the same as the rest of the bank but will still fully charge if taken out of the bank. I have been living on FLAs for over 20 years and have helped many people with their off grid systems and have worked with many battery banks.
And really, do call Crown or Surrette. I guarantee they will tell you it is not a good idea. None of the reputable solar installers I know recommend replacing weak cells with new ones. Personally my rule of thumb is put a load test on the used but good batteries and if they test above 80% it is worth it to put a new battery in it. If not, just lower the cut out voltage if needed and keep beating on the bank until it is dead, then replace the entire thing.
I would agree. That happens because the cells were all operated under the same conditions for exactly the same length of time. They all got old together. I fly airplanes and the same thing happens to the light bulbs on the plane. If the right side "nav light" burns out and I only change that bulb, a few days later the left "nav light" burns out, then 3 days later the rear one burns out. That happens because all the nav-lights are on at the same time and they run for the same length of time. So I just change all of them when one burns out since the others aren't far behind. BUT I must point out that you are talking about batteries in SERIES, which we were already on the same page about that it can't be done with series cells. If you have multiple strings of cells, you run into the same problem of the batteries all getting old at the same time since they are in use at the same time for the same length of time, thus multiple strings can go bad at almost the same time for that same reason.