I am travelling to northern Sweden next week for what will be the trip of a lifetime. Temperatures will be down to -22 and below at night when I expect to be out looking for the Aurora Borealis and -13 during the day when I will be 'adventuring'. I'm taking my reliable Panasonic FZ18 but the Desire will be in my pocket and I'll probably use it to capture some of the action in 720p. I need a view on my plans to keep the battery warm to prevent the cold from killing it. I have a silicon case and plan on putting a slim handwarmer between the case and back of the phone. Obviously they get hot, they do not indicate to what temp but do you think it'll be safe ?