When I was a young lad, my parents bought battery-operated socks. For themselves, not for me. Sometimes, when forced to sit outside in frigid midwest winter wind chills for three hours' worth of my brother's junior football games they would give me a worn burlap blanket to cover my shivering arms. And once, when my fingers were on the verge of frostbite and my mom began to worry that she would have to leave the game to take me to the hospital, I was tossed a charcoal-fueled handwarmer. Unfortunately, as soon as I opened it to see what the source of this glorious heat might be, I accidentally dropped the burning ember onto the burlap blanket and lit it on fire. After my mom beat me, she took away my staying warm privileges. The point is: 1) battery-operated foot warmers still exist, though thanks to ThermaCell they have been streamlined into rechargeable heated insoles and 2) I'm rich and famous now, so can afford to buy a pair of my own, along with a proper wool blanket, which I may or may not light on fire as I see fit.
Heated insoles have no wires or external batteries, and function via remote control for up to 2,500 hours of pedal defrost during icy winter months. ThermaCell notes the lithium ion polymer batteries embedded in the foot warmers are rechargeable up to 500 times, and adjustable to 3 temperature settings: no heat, 100 degrees, and 111 degrees.