Re: AGM battery equalisation
I'll throw in my 2 cents that it's time to start looking for new batteries. 4.5 years is a pretty good run especially if you have spent a lot of time at anchor and therefore have lots of discharge/charge cycles.
Regarding trying to equalize the batteries, check the documentation of your specific batteries. Most AGM batteries DO NOT recommend equalizing with the notable exception of Lifeline AGM batteries.
The battery with 12.3 volts is gone. Keeping that damaged battery in the bank is doing you more harm than good right now and actually can be a danger as it is likely getting quite hot when charging as well as venting excessive amounts of hydrogen gas. If I were you I would immediately take out that battery as well as the worst of the remaining 11 and run with 5 pairs instead of 6 until you can replace the entire bank.
If you just replace the bad battery with a new one, that battery will have significantly more capacity than all the rest of your "old" batteries. But your charger does not know that so when your "old" batteries are full, the new one won't even be close to full. So you will be chronically undercharging the new battery leading to iot's premature failure AND you will be overcharging the old batteries at the same time leading to excess heat and venting of gas.
Regarding "balancing out the voltages" of the remaining batteries, my understanding from my research is that in a Series-Parallel battery bank it is more important that the voltages of the 12V batteries that are connected in Series to make each 24V battery are matched as closely as possible.
See this article: https://www.coastalclimatecontrol.com/index.php/blog/192-a-true-balancing-act-battery-equalizing.html There are products on the market called Battery Balancers whose purpose is to monitor these differences and modify the charge current sent to each battery to ensure that each gets fully charged such as this one: https://www.victronenergy.com/upload/documents/Datasheet-Battery-Balancer-EN.pdf
So in your case, you said that you have the following voltages in your 12 batteries:
12.7 12.6 12.5 12.5 12.6 12.6 12.5 12.6 12.5 12.6 12.7 12.3
I would disconnect the 12.3 and the lowest of the 12.5 batteries and then make the following set of five (5) 24V batteries connected in Parallel:
Also consider picking up an inexpensive battery tester such as this one: https://www.amazon.com/BA7-100-1200-Electronic-Battery-System/dp/B0015PI7A4/ref=asc_df_B0015PI7A4/?tag=hyprod-20&linkCode=df0&hvadid=312280085431&hvpos=1o1&hvnetw=g&hvrand=4220110621597337925&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9007796&hvtargid=pla-499796143917&psc=1
Last, I am NOT an electrical engineer nor a battery expert. Most of what I know about deep cycle batteries comes from reading these sites: https://marinehowto.com/under-load-battery-voltage-vs-soc/ and https://batteryuniversity.com/ and https://www.coastalclimatecontrol.com/index.php/blog.html. They are full of great information to help you decide how best to proceed. Good luck!
SM #440 Cara
Deale, MD USA