I've read a lot of posts from Prius owners concerning reduced mileage during winter driving. I suspect a lot of this is probably due to winter fuel addatives and the inability of the car to remain warm. I haven't seen anything about the reduced efficiency of the traction battery. I know the Lithium Oion battery in my camera suffers degraded performance in cold weather. Even if charged in the nice warm house, it can't provide the current as well as it does in warm weather. Canon suggests keeping the battery in a pocket during cool weather. Might it be the same in the Prius during cold weather?