battery operated handwarmer

There is also a battery operated handwarmer. In this type, electrically resistive heating devices are used to convert electrical energy in the battery to thermal energy. Some use disposable batteries, but others are rechargeable, like cellphones, and can be used for many hundreds of cycles with the same battery. This type is not disposable hand warmer , and is more friendly to the environment.
There is alos a usb hand warmer.