This is correct. A "rested" open circuit voltage is very accurate to determining SOC however. What it is NOT capable of is telling you what "capacity" you have left in the battery.
You can have a 100Ah battery reading 12.72V rested that will supply 100Ah. You can also have a 100% full 100Ah battery reading 12.72V that only has 60Ah of capacity left in it. Both batteries are at 100% state of charge but one now has less capacity due to use.
The only true measure of capacity is to do a controlled 20 hour capacity test at 80F... The Midtronics analyzers are also fairly accurate but cost $600.00 to $2000.00... They still don't do a 20 hour "capacity" test..
I'm starting tosuspect that my 6 year old 4D Gell cells have lost some capacity. Is there a simple test I can do to determine the remaining capacity?
I do have the Victron battery monitor that you recommended, so measuring AHs removed is easy. I do leave the batteries on the boat in the winter, on a solar panel. The boat is on the hard, currently at about 30 degrees.
I don't want to do too deep a test and hurt the batteries. Not looking for super high accuracy, within the nearest 5 or 10 AHs would be good enough.
I'm thinking I could let the batteries sit disconnected for a week, noting the curent charge status (AHs already removed) and the voltage. Then I'll reconnect the negative leads and run some lights to get the batteries 30 AH down from where they were. Let them sit disconnected for a week and note the voltage.
The before and after voltage should tell me how much I've gone down in the % charge status. The rest would be simple math.
Would that work?