Joe, don’t forget the reason you changed your alternator was because of an overcharge situation due to a faulty internal regulator. That overcharged the battery for some time and may have either boiled all of the acid out, or just plain killed a cell like said before. My guess is the acid is low therefore giving low voltage esp in the cold for an extended period. Generally speaking when a cell dies in a 12v battery, you lose 2 volts give or take which will also cause this problem, but usually it would be the second you shut off your car the battery is dead.
The battery you have is a sealed maintenance free battery meaning there is no access to the cells to refill with distilled water or acid. I can’t remember, is it a NAPA battery?