[FAQ] How do I use the online GUI thermistor calculator?

In many applications the recommended Semitec 103AT-2 thermistor can be used, or a thermistor with the same specifications may be used. When the 103AT-2 is used or other thermistor with the same specifications, the temperature ranges in the datasheet can be used as a reference, assuming RT1 = 5.32kOhm and RT2 = 30.1kOhm.

If a thermistor with different specifications is required, or a hot or cold temperature is required which is not supported by the register settings, the calculator within the online charger GUI can help estimate what temperatures the charger VTS thresholds will correspond to and the required RT1 and RT2 resistors.

Example output:

Setting hot and cold temperature:

When inputting the cold and hot temperatures, it is recommended to adjust the hot and cold register values to the closest matching temperature. For example, if multiple hot temperature thresholds are available, 50degC and 60degC, and the desired hot temperature is 65degC, select 60degC from the drop-down menu. This will help prevent overly large RT1 and RT2 values.

Example selecting the appropriate threshold to match the desired hot temperature:

Custom thermistor:

When using a thermistor with different specifications than the 103AT-2, the option to manually input your thermistor specifications is available by checking the "Manually input NTC information" checkbox. Once this is selected, 3 more options are available to input. When the box is unchecked the calculator will use the AT103-2 information. The information is available in the thermistor's datasheet resistance versus temperature table.

The temperature and resistance must always match, example below, otherwise the RT1 and RT2 estimations and temperature thresholds will not be accurate.

Error estimation:

The min and max values for entering and exiting a temperature range are determine by a combination of four items:

1. The VTS range provided in the datasheet of the charger
2. Thermistor beta tolerance - provided in the thermistor datasheet
3. Thermistor resistance at 25degC - provided in the thermistor datasheet
4. RT resistors tolerance - provided in the resistors datasheet

The 103AT-2 has 1% error for beta and resistance at 25degC, which is the default setting in the tool.

Register settings:

Adjusting TS cold and TS hot will force RT1 and RT2 to be recalculated, which will cause the intermediate thresholds (cool, precool, prewarm, warm) to update to new estimated values.