Why doesn't battery charge to 12.6 volts?

I bought a Nissan Frontier 2011 just over a year ago. When cold weather set
in the battery wouldn't start vehicle. After charging on charger a few times it
still didn't hold a charge. I replaced battery. Same thing happened with new
battery. I checked and voltage was only 12.23. It was OK in warm weather. I
checked for parasitic drain and found that was very low after a few minutes,
like .008A. Running vehicle in carport voltage showed as 14.45V. That
looked good but battery still dropped to 12.23V. I hooked up voltmeter
through grill with lamp cord and noticed the voltage immediately dropped to
13,2 V and stayed exactly that when I took truck for a drive. Battery wouldn't
charge above 12.23. I finally found out about "smart" charging. I
disconnected sensor on negative terminal and took the truck for another long
drive. Voltage was solid on 14.45V and slowly dropped to 14.1V. Yeah I
thought, problem fixed. Unfortunately even with "smart" charging disabled
and an apparent high charging voltage of over 14V the battery voltage drops
down down down to around 12.3V. I'm mystified. If I disconnect battery and
charge it up it stays solid at over 12.6V for days. {I have another vehicle)
Why isn't it charging up to 12.6V via the alternator?