Hi Mogal,
Just to be clear, 2, 6vdc batteries connected in series will provide 12vdc power output. The amp/hours of storage does not double.
How are you calculating when the batteries are at "full charge"?
13.2vdc is considered a "maintenance" charge from your converter. 13.6 is the normal charging voltage. 14.4 vdc is the bulk charge mode for recharging significantly discharged batteries.
https://wfcoelectronics.com/wp-conte...eration-v2.pdf