Hi i wanted to make a little hand heater for walking in the cold. The plan is to make a little clay square and put some wire in it, fire the clay and that should give me a good temperature conductor.

I know i should use nichrome for this but ive noticed that copper wire actually heats up very fast too using a 18650 battery( i used 20 gauge bare copper wire and a 3000mah 3.7v fully charged 18650, as soon as i touched the terminals, 5 inches of copper wire got burning hot).

My question is why not use copper wire? It gets hot and hardly has any resistance, but i would put a resistor in there too just to not overheat the battery, i would also use the hand heater in bursts( not continuously), so is copper wire a bad idea for a hand heater? Why?

  • \$\begingroup\$ copper reacts with oxygen \$\endgroup\$
    – jsotola
    Jan 4, 2018 at 6:51
  • 1
    \$\begingroup\$ but i would put a resistor in there too – if you do that, your copper wire stays cold and the resistor gets hot. That's also the answer to your question. \$\endgroup\$
    – Janka
    Jan 4, 2018 at 6:52
  • \$\begingroup\$ So it would be a bad idea because it reacts with oxigen? Even if i put it in clay? What damage would it do? \$\endgroup\$
    – Donsa
    Jan 4, 2018 at 6:53
  • \$\begingroup\$ Oh and Janka, would the battery be safe if i didnt use a resistor and only used the heated in bursts? \$\endgroup\$
    – Donsa
    Jan 4, 2018 at 6:53
  • 3
    \$\begingroup\$ I'd start with a 0.33 ohm 10W resistor, it will likely be already embedded in ceramic. \$\endgroup\$ Jan 4, 2018 at 7:44

1 Answer 1


What you'll want to do is first figure out how much power you want to draw from your battery, all of this power will turn to heat one way or another.

Lets say you want 5W. Your battery is 3.7v nominal, so you need 5w/3.7v= 1.35A

To get 1.35A at 3.7v you need a resistance of: 3.7v/1.35A = 2.7 ohms

If your copper wire was 0.1ohm, the battery adds about 0.35 ohms, so you were drawing 3.7v/0.45r = 8.2A, which would produce 30W! perhaps more. That explains why your wire got so hot. And don't forget given the above assumptions 77% of that heat was being generated in the battery itself!

Ok so you need 2.7 ohms total:

  • If you use 22ga copper wire, you'll need a 145ft of it. Not practical.

  • If you use copper wire + a resistor, say a 2 ohm resistor, then almost all of the heat will be dissipated by the resistor, not the copper wire. 0.1r/2.45r = 96% to be exact. This actually would be fine if you get a nice power resistor like this one.

  • If you use 22ga nichrome wire, you'll need about 2.3 feet which is manageable.

So now hopefully you can see why nichrome wire is used instead of copper. One additional advantage is that if you heat copper wire up so that it's very hot it will quicky corrode and break. Nichrome wire on the other hand will form a protective oxide layer and will not corrode.

  • \$\begingroup\$ I deleted my answer and upvoted this instead. It essentially gives the same information as mine, except the safety aspect of shorting a Litium battery. \$\endgroup\$
    – MrGerber
    Jan 4, 2018 at 8:16
  • \$\begingroup\$ Wow thanks a lot that was VERY helpful, especially all the calculations, thanks again ill go with nichrome! \$\endgroup\$
    – Donsa
    Jan 4, 2018 at 8:48

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