12.2 is the 50% point of a battery at rest. At rest means no drain at the time and has rested long enough for any voltage drop in the battery to raise to the proper at rest voltage. Depending on temperature the time varies.
With current drain from lights, or TV, water pump, furnace, Etc you can wait until a lower voltage to charge your batteries and you will still be at 50%.
In cold temperatures you will see even lower voltage and still be at 50%.
All this being said if I have several item draining my batteries and I am watching my volt meter I let my numbers go to 11.8 before worrying about it. I have let it go below 11.5 in cold temps as well.
I do have real deep cycle batteries and so far they act as good as new.
Paul
