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# BQ34Z100-G1: Thermistor Connection for Fuel Gauge and Charging system

Part Number: BQ34Z100-G1
Other Parts Discussed in Thread: BQ25798, BQ34Z100

We have a power system which includes a fuel gauge (BQ34Z100PW-G1) and charger (BQ25798RQMR) that both have inputs for a battery pack thermistor.  What is the best way to connect our battery pack's thermistor to this system?

The BQ34Z100-G1 has the thermistor's negative terminal (THR-) connected to pin 11 (P6/TS) and the thermistor's positive terminal (THR+) connected to pin 7 (REG).

The BQ25798 uses a voltage divider from pin 4 (REGN) of the device to pin 16 (TS).  The divider is 5.23kOhms for the top resistor; 30.1kOhms for the bottom resistor in our design.

We've used zero-ohm jumpers to set the path of the thermistor either to the fuel gauge or charger.  It looks like the fuel gauge can give us a resistance reading and thus an actual temperature estimate.  But the charger seems to only tell if the thermistor is "in range" for the charger to perform its duties.

Which device gives a more accurate reading of the thermistor's value?

We'd like to use the reading from just one device to simplify our design.  For example, if the BQ34Z100-G1 gives an accurate temperature estimate, how can we use this estimate to tell the charger that value?  -or at least tell the charger that the temperature is "in range?"

Carl Olen

•     The gauge needs digital temperature as an input to calculate the temperature, it sample the voltage on the TS pin at regular interval by applying a pull down load to this pin, I think mostly this would affect the temperature sensing of the charger, looks like the best practice is to use separate NTC for charger and NTC

• Steven, thanks for your message.  Our battery pack has only one NTC.  At this stage in the project, it is not practical to add another NTC.  Which device reads the NTC more accurately, the bq34z100 fuel gauge or the bq25798 charger?  Can we use the measurement made by the more accurate device to tell the other one what's going on via an I2C message?

• It seems that the temperature measurement is more critical to the fuel gauge for properly tracking battery pack conditions, characterizing battery packs, "golden files," etc., than it is for the charger.  Can we get the NTC measurements from the fuel gauge and tell the charger the news?  This is my most important question.  Can you please get the bq25798 folks to help us answer this question?

•     Sorry, I am not an expert on Charger.

Can you help to involve a charger expert to answer Carl's question?

• We ended with three thermistors is our designs. one each for the gauge, charger, and heater.

•     Thanks for letting us know, a dedicate NTC for gauge shall be fine.