Just because your Battery says it's capacity is (example) 215AH, doesn't mean it still is 215AH.
Traditional methods to test capacity require a nearly complete discharge of the battery.
Ponder the attached discharge chart.
By monitoring the discharge current (or ah) and the time for Battery voltage to drop between two set-points,
an estimate of AH capacity without a complete discharge.
This would be done infrequently between the hours of 11PM and 3Am (when the charge controller is not busy with MPPT).
ADDENDUM
For parallel battery banks I would put forward the following example:
I have 4 banks 48v FLA each originally rated at 215AH each. That total would be 860AH. After three years the above procedure estimates the capacity @800AH . If you measure the current contribution of each bank, one could estimate the AH capacity of each individual bank.
Bank 1 30%
Bank 2 28%
Bank 3 27%
Bank 4 15%
It would be time to check ALL the connections on Bank 4, each individual battery, and then ,if no improvement, consider replacement.
I would think a monthly test would be sufficient.
Comments appreciated
You would also have to factor in the lead acid battery temperature because that may not be constant. Midnite calculates that in for it's SOC estimate reading.
I think these days most people have or will soon be moving on to lithiums from lead acid batteries.
So the way to know capacity is with counting down the ah from when the batteries are at the top charge. With lithium you can more easily see that top from the voltage and when the current drops off.
And it doesn't matter too much if they don't even get filled all the way up regularly since there are no sulphation concerns.
Larry