0
\$\begingroup\$

I'm working on a design that requires I heat a bank of 18650 batteries during their operation. The plan is to mount the batteries along with large axial 2W resistors in to a PCB and using small thermostat ICs to keep them at a set temperature. (Very similar to this cubesat battery for reference).

My question is what is the best resistor composition for this purpose and whether it even really matters much? I imagine this sort of boils down to what is the most inefficient power resistor since I want the temperature to rise as much as possible. My initial thought goes to ceramic as the entire element is heating and I see that most dedicated heating elements are made of ceramic.

Any help would be appreciated, thanks!

\$\endgroup\$
  • \$\begingroup\$ Check “hot-wire” resistors used in foam cutters ect. see here en.wikipedia.org/wiki/Hot-wire_foam_cutter Also called nichrome wires. \$\endgroup\$ – pnatk Feb 13 at 1:02
  • \$\begingroup\$ Efficiency is irrelevant. What temperature do you need to achieve over what mass of material? (in the question please, rather than comments to answers). For batteries, too much temperature is fatal, so avoid hot wire. Maybe SMD resistors on a thin PCB with a copper layer to spread the heat evenly. \$\endgroup\$ – Brian Drummond Feb 13 at 11:01
4
\$\begingroup\$

All resistors have the same efficiency, they convert the dissipated power to heat. The answer depends on whether you want to heat air or a surface. A resistor that is mounted to the chassis with screws will transfer the heat to a surface more efficiently. A resistor with poor contact to the surface, will mostly heat the air.

Chassis mount resistor: enter image description here

https://www.digikey.com/product-detail/en/stackpole-electronics-inc/KAL25FB1R00/KAL25FB1R00-ND/1646178

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ In a previous design I used that exact resistor actually for heating the ambient air in the enclosure. The intent now is to use a series of lower wattage resistors thermally epoxied to the batteries to heat them conductively, similar to that cubesat battery I linked to, with the idea being less heat lost to the surrounding environment. \$\endgroup\$ – maxmclau Feb 13 at 4:49
2
\$\begingroup\$

If this is a Cube-sat application, For aerospace I recall the two major concerns were reliability and efficiency.

For reliability in a low air density environment and also for efficiency, you want the injected heat / lost heat ratio to be high, I expect. Thus the ratio of thermal resistance ratio for battery insulation to heater conduction to batteries must be high. This way the hotspot on the battery or thermal gradient must be low yet low energy lost in the insulation and the resistor not exceeding some conservative temperature, such as 100’C with some rate or range limits.

It is normal for larger Alum-clad power resistors to use some optimal material inside with proprietary thermal conduction. Wirewound cement resistors are also popular. Depending on the power needed, you might also consider and array of copper clad FPC circuit board with SMD resistors bonded to batteries with SMD thermistor for regulation on board. I’ve used this method inside styrafoam for making a mini-oven out of a HC style crystal holder to operate at 70’C with SMD heater.

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ It's a high altitude balloon payload - similar restrictions to a cubesat though. I like the idea of a copper clad FPC. I was initially looking into off the shelf flexible polyimide heater strips which operate under a similar principle and perhaps I'll return to this idea as it seems like it'd avoid the problem of heat dissipation. \$\endgroup\$ – maxmclau Feb 13 at 4:53
  • \$\begingroup\$ as I recall we had one board with 2oz Cu on FPC . Board shop had some warp issues with unbalanced copper though. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 13 at 9:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.