Ardica's military pack, now in testing, tucks hydrogen fuel cells and lithium-ion batteries into a flexible case. The Moshi uses only batteries, since fuel cells aren't widely available. But the layout is similar: Seven 2.75-inch batteries are linked inside a foam sleeve, so the 11.3-ounce package can flex at the joints. It slips into the jacket's back and holds enough power to simultaneously drive heat-creating conductive yarn and send current to a USB cord in a pocket. You get about nine hours of heat or 11 iPhone charges before you have to plug it in.