I'll dive right into it.
A project of mine involves a glider in a cold climate. Past the point of gliders not being efficient in cold climates in the first place, I want to keep the batteries in the glider warm, and decided that heating them up directly would be one way to tackle the issue.
I want to heat the batteries, 18650 Li Ion rated at 7.4V 2600 mAh, to 25 C (ideally) for around three hours. Is it possible to do this without drawing more than 500 mAh? I would prefer to build the "heater" and do the soldering myself but if there's a product that efficiently does this then I wouldnt be opposed to just outright buying that as well.
The actual question, or TL;DR:
1.) Is it possible to heat an aluminum surface, 3 in^2 to 25C for three hours while drawing less than 500 mAh?
2.) if so, what kind of resistors/any other items would I need?
I would of course have an insualted enclousure for the batteries to sit in with the aluminum surface. I just need the surface itself to reach 25C.
PS:
I saw this video as well: https://www.youtube.com/watch?v=xIg2R9327bg
It basically works by short circuiting the battery, and draws 1 A. Is there any way to lower the amp draw? What would I need to add to the circuit? Also, is it wiser to straight up use resistors or to use nicrome wire to heat it up? Weight is also an important factor…
Best Answer
The specific heat of an 18650 LI Ion battery is 0.83 J/gK. Weight is 46.5 g. To convert mAh to Joules, you need to know the voltage. If you are working at 3.6 volts, 500 mAh = 1.8 watt-hr. Since a joule is 1 watt-second, multiply by 3600 to get 6480 joules. You need 0.83 * 46.5 or 38.6 J for each cell to raise the temperature 1 degree K (or C). So 6480 J would increase a single cell's temperature by 167°C in a perfectly insulated environment. The addition of an aluminum surface just adds specific heat, so you just need a little foil to prevent a hot spot. Just insulate the batteries well and put in whatever type of resistive load makes your batteries happy. Efficiency will be a function of the insulation; essentially all of the energy will go into heat. Of course if you use a small load it will take longer and your imperfect insulator will cause a loss of some heat, so you don't want too small of a load. Also don't let the batteries get too hot.