Lithium cells or packs should be protected from the direct cold.

First, keep the pack initially warm in your pocket or a heated room. I typically set my planes up indoors or in a warm car before going outside. If the cell is directly exposed to the cold like in the Wattage Micro Flyer, cover it with a thin piece of foam or Velcro strip.

The cold will make your plane seem like it lacks power. This is a great environment to modify a plane or find a plane that works well. My Zagi wings or Graupner Terry fly very well in the Winter and easily land in the snow.

I specifically bought my Hobbico FlyZone Ventura RTF package so I can take a cheaper transmitter and plane out in the Winter snow for some fun. It flies great in the Winter without any modifications.