You can layer up. You can put on the thickest, warmest coat you can find. But sometimes all that still isn't enough to keep you warm. For those occasions, you need the Brunton Heatsync Vital. This personal heating system is designed to rest against your base layer, and relies on a 5V/2.1Amp USB power source — like the company's Power Bank — to help it keep your core warm in the most extreme conditions.