I find that if it is cold out, and I must be outside for longer periods of time, that the “hot shot” thingies that go into your gloves and boots are best. There are no batteries, you just crush the little packets so that the contents mix, and they stay warm for a LONG time, like 12 hours long. “Cold” here is -30C. I find that if you can keep the extremities warm with these things, the rest of you is also warm. Especially if you are moving around, working. I have used these things when we go ice fishing, which entails sitting still on a frozen lake for a few hours. If it is cold out, about the last thing you want to do is overheat yourself, and sweat. Because then you will be really cold when wet.