A thermocouple, basically, attached to the side of a twigs-'n'-sticks backpack stove. The tthermocouple, a type of ransducer, is just a large version of the temp sending unit in a gas stove or furnace. The simplest form is two wires of different alloys that are in physical contact where the heat is applied, electrons flow between them in one direction when they are heated, producing current (in most applications, enough to operate the temperature guage). There are particular combinations of alloys that the engineers who design these use, they have to not only maximize the flow but both metal have to be relatively corrosion-proof and easily withstand the heat range for the source that will be applied.
Not so sure about this, they applications demand a pretty high output for a thermocouple, and they would also require a pretty prolonged time with a sustained heat to accomplish the charging, even if the output was up to it.