I too have never had problems keeping my core warm; it's hands and feet that occasionally have problems. Of course it's generally pretty mild out here in the Pacific NW, but I did grow up in Wyoming, walking to school in occasional -40F (also -40 in Celsius) temperatures. Yes, I know, all of us old folks have such goodies to report. There was a school bus, but after consistently waiting for 10-15 minutes during every cold spell, I decided that I was warmer walking! Actually, this still goes on; my grandson has to walk a mile each way to his junior high school in the Seattle suburbs.

Because of the limited life of batteries, it seems to me that relying on them, instead of on adequate insulation, for warmth in cold weather would be extremely dangerous.

The last thing I want are more batteries (which in winter have to be kept from freezing and therefore have to spend the night in my sleeping bag) or, horrors, wires, no matter how thin, next to my skin!