I tested my MagicShine by putting the battery pack in the freezer, which was running at about 10*F. Run time was diminished from 3 hours 10 minutes to 2.5 hours. Or thereabouts. It shut off when I wasn't paying attention, sometime between 2:15 and 2:45.

I'm going to check the temp of the deep freeze in the basement, since I really want to know the run time at -10 to -20*F, since I do ride in the dark in those temps. But with the length of the cord I could just put the battery in a pocket when it's that cold.