# Calculating resistors to go with a thermistor

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.

Thermistor datasheets: Option A, Option B

(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.)

Thoughts?

• What are the target Rhot/Rcold resistances for your two options? – Spehro Pefhany Aug 9 '18 at 12:54

Lucky you, I just had to figure this out for a similar part. Can we attach a spreadsheet here? If not, just assume your Rc is in cell A2 and Rh is in cell B2. Due to the quadratic, there's two solutions for R2:

=(-19*A2-19*B2+(19*(19*A2^2+19*B2^2-38*A2*B2-4*B2^2*A2+4*B2*A2^2))^0.5)/2/(B2-A2+19)

=(-19*A2-19*B2-(19*(19*A2^2+19*B2^2-38*A2*B2-4*B2^2*A2+4*B2*A2^2))^0.5)/2/(B2-A2+19)

(Those can be copy/pasted directly into Excel.)

For each of those, you can calculate R1 (B5 being the cell with an R2 solution):

=5-B5*B$2/(B5+B$2)

Hopefully, one of the solutions will have both resistances positive. If anyone has an idea how to attach a spreadsheet here, I'll do that.

• Looks like Rc (A2) and Rh (B2) should be entered in kOhms. – user159625 Aug 9 '18 at 14:48
• You can use a service like uploadfiles.io or dropbox (set to public). Both are free (maybe with advertisements and metering of data). – Spehro Pefhany Aug 9 '18 at 18:23
• @user159625 Yes, they are in kOhms. – Cristobol Polychronopolis Aug 10 '18 at 12:07
• @SpehroPefhany, there are a lot of options, I don't know the preferences of the OP. It would be nice if there was a default option on SE. – Cristobol Polychronopolis Aug 10 '18 at 12:09