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BQ27427: The accuracy of temerparue measured function as external NTC on BQ27427

Kevin Chin
Prodigy 110 points
Part Number: BQ27427

Hi Sir,

         I'd like to use external NTC on BIN pin of BQ27427 for cell temperature moeasurement. 

I tried to use a 10K (1%) value resistor, connected between BIN and VSS on EVM at room temperature .

By Bqstusio the result is 18.65°C, but the internal temperature is 24.45°C.

Is this tolerance what you expect?　

And I found this offset could be fixed by "EXt temp offset". It means that we need to do cailbration process for each gauge. right?


    

  • 0
    TI__Mastermind 24931 points

    Hello Kevin, 

    Please see the following section from the BQ27427 datasheet. 

    What NTC are you using here? The gauge's default configuration is for the Semitec 103AT type thermistor. 

    Regards, 

    Jonny. 

  • 0 in reply to Jonny
    Prodigy 110 points

    Hi Jonny,

                  103AT-2 NTC will be used in our project.　

    So I just checked the accuracy of gauae at 25 degreeC on EVM, using 10K resistor (1%).

    And the result is what I mentiond in the previous information 18.65 degreeC vs 25 degreeC) 

     

  • 0 in reply to Kevin Chin
    TI__Mastermind 24931 points

    Hello Kevin, 

    Which thermistor do you have the gauge configured to use? Please ensure that you have the gauge configured to use the external thermistor (see image below taken from the bq27427 TRM). 

    Regards, 

    Jonny. 

  • 0 in reply to Jonny
    Prodigy 110 points

    Hi Jonny,

                 The OpConfig[TEMPS] setting  is 01. 

  • 0 in reply to Kevin Chin
    TI__Mastermind 24931 points

    Hello Kevin, 

    I have discussed this with the team, and it is likely that the behavior is due to the thermistor coefficients or the external temperature offset. Please refer to the following taken from the BQ27427 TRM. 

    Regards, 

    Jonny. 

  • 0 in reply to Jonny
    Prodigy 110 points

    Hi Jonny,

                 We could use a internal temperature sesing result as reference to calculate this ofset from external sensing result.  

    By this offset valur to do calibration to compensate this offset for each gauge. right? 

    Does this offset (5.8 degreeC) meet your expectation? I don't see any specification value about external sensing part.   

  • 0 in reply to Kevin Chin
    TI__Mastermind 24931 points

    Hello Kevin, 

    When you increase/decrease the temperature in the environment, is the gap between what the internal thermistor reports and the internal thermistor reports the same? If they are the same, then you should be able to compensate this on the offset. If they are different, then this could mean that it is the thermistor coefficients. 

    Regards, 

    Jonny. 

  • 0 in reply to Jonny
    Prodigy 110 points

    Hi Jonny,

                  Do you mean that first check the temperature gap between internal and external thermistor result when different temperature?

    I found there is always same temperature gap (5.8 dgree C) between internal and external thermistor result when increasing or decreaseing temperature. 

    If this is the thermistor coefficients problem, how could I fix it?  

  • 0 in reply to Kevin Chin
    TI__Mastermind 24931 points

    Hello Kevin, 

    Since the gap is the same between the internal and external thermistors throughout different temperature ranges, then you should be able to compensate this difference by changing the offset.  

    Regards, 

    Jonny. 

  • 0 in reply to Jonny
    Prodigy 110 points

    Hi Jonny,

                  Thanks for your information. 

    I could say the result of  5.8 degree C temerature gap is in your specification. right?　

    If so, this is the hardware limitation about temperature measurement via external. it means that may this gap be happened on MP version or larger than this one?   

    Whatever, we have to do cailbration process to check the accuracuy if we want to use external measurement function. right?

  • 0 in reply to Kevin Chin
    TI__Mastermind 24931 points

    Hey Kevin, 

    As long as the offset does not exceed what is listed in the BQ27427 documentation, this should be good. 

    I am not sure what is being asked in your last questions here, can you please elaborate?

    Regards, 

    Jonny.