Right kind of resistance wire for near-skin heater?

I'll be up-front and say I know jack squat about electrical engineering aside from "conductors heat up when you run current through them."

I'm looking in to building a low-voltage (~12-24 volts) personal heater that I can attach to the inside of a jacket; basically a small battery-powered electric blanket with no control system other than just unplugging the battery when it gets too hot and plugging it back in when it gets too cold. I'm not sure how possible this is to do safely though.

My main question is, what should I use for the heating coil? I was hoping to have a long wire that zig-zags over most of the back of the inside of the jacket. Kanthal wire is really cheap right now 'cos of all the people wanting to build vaporizers; is that an appropriate material? If so, what specific mixture and gauge should I be going for?

Looking at the specifics it seems like it's meant for extremely high temperatures, whereas I want something that stays around 30-40 degrees Celsius on low voltage. I can't seem to find anything that says it's designed for that kind of application, though. Am I barking up entirely the wrong tree?

Is this a practical idea at all?

• This is more of a thermal problem or mechanical problem. The temperature of the resistor will be determined by the size and what materials are around it. Kanthal wire is low resistance so it is going heat up rapidly to what temperature, no one knows. – Voltage Spike Jan 12 '17 at 19:54
• Consider using "positive temperature coefficient" PTC flexible sheet heating elements. As they require no additional regulation, are flexible, light weight, designed for no hotspots, energy efficient, etc.. Purchase them for your target temperature. If part of the jacket loses more heat than another part, it will deliver more heat to the needed area. – Optionparty Jan 13 '17 at 2:55
• A low price source may be a used ptc automotive seat heater from an automotive parts yard or online. If your battery were rechargeable, you might build an auto cigarette lighter plug, to charge the battery and initial heat to the jacket. Good luck, let us know how you do. – Optionparty Jan 13 '17 at 3:19

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.

• This is not the right way to calculate the power needed. It's not about raising the temperature of flesh, but in keeping up with the heat loss thru the jacket. – Olin Lathrop Jan 15 '17 at 22:09
• @OlinLathrop hm, that's a reasonable way to look at it. I just assumed that in the end, you wanted to heat a piece of human, and could assume the jacket to be well-isolating, but I see the inherent shortcoming of that (because if you had a well-isolating jacket, and are not acutely sick and/or severly malnourished, you should be warm in very short time) – Marcus Müller Jan 15 '17 at 22:11

Your desired temperature is low, but the wire is small so will get significantly hotter than the overall temperature increase. Nichrome is probably a good choice.

However, the real problem is the total energy required and the size battery that will take. Maintaining let's say 10°C additional temperature rise over the surface area of a jacket will take some power. You'll have to measure that. My guess is it would take 10s of W.

Another problem will be the temperature of the wire. You'll probably wrap the wire in a zig-zag with the spacing being much larger than the wire's diameter. This means the wire itself will be significantly hotter than the average temperature rise. That could be a problem if it directly touched the skin, or it could melt, otherwise damage, or even catch the insulation on fire.

Yet another issue is that you don't want the inside of the jacket to be 40°C. That's above body temperature. That will get uncomfortable fast and cause sweating, which is really bad in a winter situation.

Go get a better jacket instead.

• +1 Especially for the last sentence. Since you wouldn't want to waste power and spread heat outside the jacket, you will end-up coating the heating elements with very good thermal insulator on the outer side. That is a better jacket. Once you have good insulation, you won't need internal heating anyway, unless you are trying to help someone who suffers from hypothermia. – Lorenzo Donati -- Codidact.com Jan 12 '17 at 19:33
• Such a device could be useful only in very extreme environments, where space-suit-like clothes are almost mandatory (deep sea workers/explorers, high altitude jet pilots, etc.). I don't think even arctic expeditions needed such complications (even during arctic winter), but I may be wrong. – Lorenzo Donati -- Codidact.com Jan 12 '17 at 19:33
• @Lorenzo - You might be wrong. Arctic explorers probably don't need it because they are selected for good health : quite a few non-Arctic-explorers have health problems where a little help avoiding hypothermia could be worthwhile. 100Wh or so from a Li-Po "flight pack" giving 50-100W for an hour or so could be useful. But Olin's certainly right that 40C is too much - probably so is 30C except for a few minutes boost. – Brian Drummond Jan 12 '17 at 19:47
• @BrianDrummond Yep, you have a point here. Anyway, I agree that the OP's idea has value as a sort of "medical" equipment (i.e. emergency "thermal blanket" ). I've the impression, however, that the OP wants to create a "thermally boosted jacket" as a commodity device, which seems a bit over-complicated to me. – Lorenzo Donati -- Codidact.com Jan 12 '17 at 20:27
• Thing is, the coat keeps me warm most of the time, especially when I'm working my chest muscles, but for the first twenty minutes or so that I'm wearing it or if I have to stop moving I'm uncomfortably cold. I was hoping to get a boost during those times. – Schilcote Jan 12 '17 at 21:08

Totally boring but https://www.makita.co.nz/products/model/DCJ200 would solve the problem. It will heat for a couple of hours off an 18v rechargeable pack.

• Boring and expensive, those things are hundreds of bucks and don't even come with a a battery! I worked out a way to run a 12-volt electric blanket for use in cars off a battery pack and that only cost \$100 in total; I would've gone for it if I could find a blanket that was small enough. – Schilcote Jan 15 '17 at 15:33