Electronic – How to correctly estimate heat output for heater wire

If your heater is to be directly mains-powered you might be better off to buy a commercial heater as it will be properly designed and insulated. Popular types are band, strip, cartridge and calrod.

You can design the heater take whatever current you happen to have available using a given resistance of wire, however the length of wire will be fixed at something that is probably inconvenient, and the voltage may not match what you have available.

For example, if you design it for 1A you will need 65 ohms (and 65V), which is 65\$\Omega\$/8.286m\$\Omega\$/m = 7.8km of AWG14 wire. Not very practical.

If you design it for 12V power, you would need 5.4A and more like 260m of wire (still a lot).

At 3.3V you would have 20A but you'd still need 20m of wire. That's about the lowest voltage supply cheaply available (within the range of a 500W ATX supply).

That is one reason why heaters are normally made with nichrome, Kanthal or other high-resistance (and refractory) alloys.

It probably is feasible.

But: you need to realize that even for electricity, conservation of energy applies! So if you want to produce 10,000 Ws (wattseconds) = 10 kJ of heat, you need to have that much energy in your battery.

So, in your place, I'd start by calculating what amount of heat you want to produce. That's really easy. Calculate the amount of material (ie. flesh) you want to heat up, multiply that by the specific heat of the material (use the specific heat of water, flesh is mostly that stuff), and then you know how many Joule you need. One Joule is 1 W · 1 s.

Then divide that by the time (in seconds) you want to allow the heating to heat up that much material, and you've calculated the power (in Watt) necessary to heat up that amount of material that fast.

Now, I don't believe you know nothing. You've probably heard of 1 W = 1 A · 1 V. So if you have a 12 V battery, in order to produce 1 W, needs to make \$\frac1{12}\text{ A}\$ current flow. If you need 120 W, you'll need 10 A. It's that simple.

Now that you know how much current you need to spend, you can calculate two things:

1. how long your warmer should last with a single charge. Most batteries are rated in "Ampere hours", Ah, so take one battery that you think would be of acceptable weight and size, and figure out how long it will be able to source that current.
2. how much resistance your wire needs to have, because, Ohm's law, U = R · I -> R = U / I (the resistance is the ratio between voltage U=12V and the current I). Heater wire has a fixed resistance per length. You can thus calculate the length of wire necessary.

This doesn't address things like maximum power impedance matching etc, but it does in theory give you a good idea of what your system needs to look like.