NTC thermistors tell a laptop how hot its battery, charger and regulators are. Their resistance falls as temperature rises, along a curve defined by two numbers on the datasheet: the resistance at 25 °C (R25) and the B value. This calculator converts in both directions and shows the voltage the EC or charger IC sees when the thermistor sits in a divider with a pull-up resistor.
How to use it
- Enter R25 and the B value from the schematic or datasheet. A 10 kΩ, B 3435 or 3380 part is common, but check your board.
- Choose the direction: temperature to resistance, or a measured resistance to temperature.
- Optionally enter the pull-up resistor and its supply voltage to see the sense voltage.
- Use the table to compare a reading against the whole curve.
The formula
R = R25 × e^(B × (1/T − 1/298.15)), with T in kelvin (°C + 273.15). Rearranged: 1/T = 1/298.15 + ln(R ÷ R25) ÷ B.
The B-value model is accurate to a degree or two over the usual range. Datasheets give more precise tables when exact temperatures matter.
With the thermistor from the sense pin to ground and a pull-up resistor to Vref, the pin voltage is Vref × R ÷ (R + Rpull) — the same arithmetic as Voltage Divider Calculator.
Worked example: a battery that will not charge
A laptop refuses to charge and reports an implausible battery temperature. The pack's thermistor is a 10 kΩ, B 3435 part, read through a 10 kΩ pull-up from 3.3 V. At room temperature the thermistor should read about 10 kΩ and the sense pin about 1.65 V. The pin actually measures 3.29 V — close to the full pull-up voltage. Converting back, that corresponds to an extremely high resistance, a "temperature" far below freezing. The thermistor line is open, and the charger refuses to charge a battery it believes is frozen. Testing the part itself is covered in How to Test a Thermistor with a Multimeter, and the component under ntc thermistor.
FAQ
Where do I find the B value?
In the thermistor's datasheet, or in the charger IC's reference design. Schematics often give only R25; common laptop values are 10 kΩ with B around 3380–3435.
Why does my reading not match the table exactly?
The board temperature is rarely exactly what you assume, and parts have tolerance. A few percent difference is normal.
Does a PTC thermistor follow this curve?
No. PTCs rise sharply above a threshold and need their own datasheet curve.