You’ve seen it before: you pull your phone out of your pocket after an hour of skiing only to discover that the battery has gone from fully charged to 50 per cent. The lithium-ion batteries that our cell phones depend on aren’t designed to withstand extreme temperatures. Apple suggests that iPhones perform best between the range of 0 and 35 C. In frigid temperatures, your phone can randomly shut off, which doesn’t help in emergencies. For cold-weather adventurers planning to head to the mountains this winter, here’s how you can protect your phone and prolong its battery life when you’re in the cold.