A little more info for this- A dead battery that has already frozen is slow to charge while still frozen. A battery that was drawn down from a charged state below freezing will charge fairly quickly. I deal with dead, frozen batteries every winter in temps down to -30f, and even a dead battery frozen overnight will charge with it still in the vehicle outside in subzero temps, it just takes longer.
The original owner of my bike put a battery tender on it in an unheated garage (battery still in the bike) every winter, and the original 2006 battery was removed while still working this past spring. Last winter I stored it in my heated basement with no charger, and come spring it started right up without charging.