I am working on my first etextile project: heated gloves. To experiment I sewed a U shape with extra thick stainless steel thread (1.4 ohms per linear foot) into some felt. One end of the U is connected to the positive terminal of a 9v battery and the other end of the U is connected to the negative terminal of the battery using alligator clips.
My thread is heating up a little and my battery is heating up a lot. What am I doing that would make the battery heat up like this?
jcorpey:
My thread is heating up a little and my battery is heating up a lot. What am I doing that would make the battery heat up like this?
Probably drawing more current from the battery than it was designed to give. 9V betteries aren't designed to give much more the 100-200 mA. More than that and the internal resistance of the battery drops a lot of voltage. That's power dissipated, and thus heat, in the battery.
To answer your question: My conductive thread was about 24" long. But after reading your comment I was able to do a bit more research. I was A) As you mentioned, drawing more current that my battery could handle, and B) not producing enough power to convert into heat.
I now understand more of the relationship between Power, Current, Voltage and Resistance (P=IV and V=IR). However I’m having a hard time figuring out how many watts of power I want to produce to get my thread to heat up. It seems to me that I would need a significantly more powerful battery to heat a pair of gloves to a point that would keep my hands warm in the winter. Or maybe another type of material? Nichrome wire (not sure if this would bend with finger movement)?
jcorpey:
It seems to me that I would need a significantly more powerful battery to heat a pair of gloves to a point that would keep my hands warm in the winter.
Exactly ! I don't have a good number for the power needed to warm hands. It really depends on how well insulated the gloves are. A lot of insulation and the power (heat) delivered to your hands via normal blood circulation is enough. Less insulation means heat loss and a temperature drop, unless supplemented with more (external) power/heat. I suspect a good sized LiPo might work w/a good set of gloves. Then again I've never had gloves that needed heat. You might look at some of the gear snomobilers use, to get some idea of watts needed (pun intended).
Also remember that electrical power turned into heat will warm you, but for how long. Power * time = energy, aka battery “capacity”. Ideally a nominal 3.7V LiPo, rated at 2000 mAH, means you could draw 2000 mA (= 2A) for 1 hour at the 3.7V. That’s 3.7V * 2A = 7.4W for 1 hour. Or less current for longer. Or more current for shorter. So you need a battery that can not only supply the needed power but also for the needed time.
Then you need a “wire” than can handle the current needed. You didn’t say what thread you used. The “thick” thread SFE sells is supposed to be 4 ohms/ft. Your 2 ft would then have a resistance of 8 ohms. Neglecting any other resistance in the circuit or battery, connecting that up to the LiPo above would draw (on average) 3.7V/8ohms = 463 mA and dissipate ~ {EDIT that’s 1.7 here, not 3.7} 1.7W. The current is less than the 4A max (2C rating) that the battery could safely supply so that’s OK for the battery. Is 1.7W enough to warm your hand in an insulated glove ? I dunno. Will the thread overheat and melt ? I’d need to know the insulation provided by the glove (but probably not, still it’s a question to be asked and answered).
Hey I’ve got one more question. If my battery has a capacity of 2amps. Can I use less than that by adjusting the resistance? I guess a more accurate question is, does my 2amp battery always have to produce 2 amps? Would reducing the current increase the longevity of the battery?
jcorpey:
If my battery has a capacity of 2amps. Can I use less than that by adjusting the resistance?
Yes. You can always safely draw less current out of a battery, power supply, etc than it's rated max current. This will increase the life of the battery.
I just wanted to let you know that I have successfully completed a pair of re-chargable, battery powered, heated slippers. I am now working on heated mittens that i will post on instructables. Thanks again for your help!
