Why is voltage level affected by low temperature?
Batteries work on chemical reactions.  Chemical reactions slow down at lower temperatures.  This slow down shows up are a reduction of capability.  I've been looking into this because the nimh batter pack I use to ride my bike to work is dying.  Most bike lights now are Li-on.  Since I ride in the winter this is an issue for me.  There doesn't seem to be an exact agreed upon reduction, but it seems like the consensus is -10% at 0C and -20% at -20C.  If you do a search you will find graphs that show the change in voltage, resistance, and capacity as a function of temperature and discharge rate.  The problem is the graphs very from site to site.