That battery looks a bit heavy to me, but depending on how far you have to carry it, and how cold it is, I can see how this gizmo from Berto could make the difference between a really nice outing and a really miserable one. And it’s dead simple to make: Find a 12V electric blanket, cut a hole for your head (being careful to avoid the heating coils), reinforce the hole with some sewed-on cloth, and wire to a switch and battery to be carried around in your backpack. Berto used a sealed lead gel battery, which could, of course, be substituted with a much lighter (and more expensive) lithium-based unit.

Make: Projects — Battery-Heated Poncho