I don't know about you guys - But the over night temperatures around here get to be -20 to -40 degrees.
Something I've notice time, and time again is that ~ When I start my vehicle in the A.M. after a cold night - My voltage sags around 13.8 - But once I let the engine warm up for about 20 minutes, It goes back up to 14.8+
Not sure with all these mixed results - but it seems like this problem is really dependant on the type, specs, and age of your battery -