You guys are doing nonsense. Buy a voltage regulator and plug it in instead of the battery, as I did.
https://aliexpress.ru/item/4000259403990.html?spm=a2g0s.9042311.0.0.264d33edswGqmU&_ga=2.143500917.1742143978.1617214605-1049604428.1586439898&sku_id=10000001054452173Instead of a battery, I installed a downconverter 4_5-24V to 0_8-17V 2A at MP2315 500kHz.
I set the lowering to 3.9V (from 5v, received from the main USB Male connector (mico-USB, Type-C), by shorting the 5V and 12V jumpers on the converter).
In this case, depending on the smartphone model, the Flir One Pro application always shows the charge level of a non-existent battery at 70-82%.
I took + 5V from the control patch on the thermal imager board to the right of the USB Male connector cable (mico-USB, Type-C).
A pleasant bonus is that when you turn on OTG in a smartphone (if you need to turn it on in principle), the thermal imager turns on immediately, there is no need
press the power button of the thermal imager.
If you close jumpers 5V and 12V, the output will be exactly 3.9V
When I did all this, I did not take pictures, so there are no photographs. But everything has been working fine for a month already 3. The thermal imager does not heat up inside, and the current consumption from the smartphone is minimal.
No resistors are needed to simulate a thermistor. The main thing is that after revision, never connect voltage to the Type-C charging port.