Mapping Charger Temperature Thresholds to the Actual NTC Network

A battery charger temperature input interprets an electrical signal, not temperature directly. The thermistor curve, bias method, and external resistors determine which physical temperatures correspond to its thresholds. A nominal resistance value alone is not enough to establish the intended charging behavior.

Start from the battery requirements

Obtain the permitted charging conditions from the battery manufacturer, including any required changes in current or voltage with temperature. Then read the charger’s temperature monitoring behavior and available settings. Do not assume that a generic temperature profile is appropriate for every cell or pack.

The TI BQ25882 data sheet describes thermistor qualification using a TS input and an external network. The relationship between its electrical thresholds and temperature must be calculated for the chosen thermistor and resistor arrangement.

Use the full resistance curve

Record thermistor resistance at the required temperature boundaries using manufacturer data. Include resistance tolerance and temperature characteristic tolerance. Retain the manufacturer’s resistance table or equation used for each boundary calculation. Two thermistors with the same nominal room temperature resistance can produce different voltages elsewhere in the operating range.

Calculate TS voltage or ratio using the actual bias source and network. Include resistor tolerances and the charger’s threshold tolerance. If the threshold is ratiometric, preserve the specified reference relationship rather than substituting an unrelated supply.

Check placement and fault states

Verify that the thermistor measures the intended battery location. A board mounted sensor near a warm regulator may report a different temperature from the cell. Use the pack manufacturer’s prescribed sensing arrangement where applicable.

Evaluate open circuit, short circuit, disconnected pack, and unpowered conditions according to the charger’s documented response. Do not assume every TS fault simply disables charging; devices can assign additional meanings to input ranges or use configurable behavior.

Verify thresholds without stressing the battery

Use a suitable resistor substitution or thermistor simulator to check electrical transitions and reported status before testing physical temperature behavior. Confirm both directions of each boundary, including hysteresis and any qualification time. Keep the actual charging path within the battery’s approved conditions throughout testing.

Then verify the installed sensor response using a controlled temperature procedure appropriate to the pack. Save the thermistor part, resistor values, tolerance calculations, threshold settings, and measured transitions. The review should demonstrate that the physical battery temperature produces the intended charger state with margin. A correct TS voltage at room temperature establishes only one point in that mapping, not the entire protection window.