gecho

The heating rope is 3 mm carbon fiber, that I got from https://www.carbonheater.us/ . The minimum order size / cost definitely makes it more expensive that a premade heating pad.

The rope is made up of 12 smaller ropes, for my heat pads I used 2 parallel circuits with 4 of the smaller ropes instead of the full rope. The full rope has a resistance of about 21 ohms per meter, the individual strands about 228 ohms. The full rope uses more power than I need in a particular application. Its a bit tricky tuning the heat output, in theory the rope and strands will achieve the same temperature at a given length. But with less strands you can pull heat out fast enough it never reaches that temperature. I've started calculating the estimated wattage to give me a rough idea of how warm a particular design might get at a particular voltage as I have 5V, 7.4V and 9V batteries that I can use to power them.

I recently started using 9V with a USB C PD power banks and a USB PD decoy. More consistent heat output than 7.4V batteries which start around 8.4V and drop over time causing the heat output to drop. USB delivers a constant voltage.