This battery regulator chip I'm using has a pin for a 10kΩ NTC thermistor to let it adjust the charge rate in response to the battery's temperature. You're supposed to choose two resistors to set the range of temperatures that the battery will be charged at. If the battery temp gets outside this range, charging stops.
The equations to choose the resistors are:
24kΩ = RT1 + (RT2 * Rcold) / (RT2 + Rcold)
5kΩ = RT1 + (RT2 * Rhot) / (RT2 + Rhot)
After a moment of juggling, I get this equation to solve for R2:
((RT2 * Rcold)/(RT2 + Rcold)) = ((RT2 * Rhot)/(RT2 + Rhot))+19000
Plugging that into Wolfram Alpha, I get solutions, but the values are negative!
Datasheet for the power regulator - The thermistor math is the last note on page 28.
(These are the two options for thermistors from the Seeed Open Parts Library. I was hoping to stick to one of these two, even if neither was the recommended 10k.)