Review on TriPole Hand Warmer Rechargeable 5200MAh Electric Handwarmer Reusable USB Pocket Hand Heater Portable Power Bank, Warm Gift For Winter Outdoor Sports Women Men, Double-Sided, Fast Heating, 2 Levels by Eric Fuego

Heats up immediately. Its super slim and can be wedged between layers. I put a badge clip on the end of it and wore it around my neck to where it was chest or stomach height and it feels so cozy and warm. I made a sleeve for it to be in cause it gets quite warm and feels nice having a soft layer between that and your bare skin. Only last a couple hrs but it came in clutch several times last winter. I can't complain.