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Original question:

BQ34110: Need the temperature coefficient calculator for BQ34110

BQ34110: External thermistor temperature value prediction

Chris Mayo
Prodigy 10 points
Part Number: BQ34110

Hi,

I have seen several threads where people have asked for customised coefficients for non-standard thermistors for the BQ34110. My enquiry is slightly different, I do not currently have hardware to test the thermistor, but I need to predict the temperature reported for specific resistances throughout the range of the thermistor. This is assuming using the recommended Semitec 103AT thermistor. I have tried modelling it using the conventional thermistor Beta equation but this gives errors at the low temperature end. Clearly the coefficients given in the reference manual indicate a different equation is used, presumably a polynomial. Apparently it is not the normal Steinhart-Hart type polynomial as the coefficients do not work in that. My question is - please can you tell me what the equation is that uses the coefficients so I can accurately model the temperature reporting?

Thank you

Regards

Chris Mayo

  • 0
    TI__Guru 57747 points

    Hello Chris,

    The default coefficients are matched to the recommended thermistor. It is a polynomial but the details have not been published

  • 0 in reply to Shirish
    Prodigy 10 points

    Hi Shirish,

    Thankyou for the quick reply. When you say not published, do you mean it is confidential or proprietary and you cannot tell me, or do you mean it is embedded in code and you have no way of finding out what it is?

    Regards

    Chris 

  • 0 in reply to Chris Mayo
    TI__Guru 57747 points

    Hello Chris,

    It is something that TI decided not to disclose. It could be confidential or proprietary or just to keep things simple for the end user. 

  • 0 in reply to Shirish
    Prodigy 10 points

    Hi Shirish,

    Unfortunately it is not working out to be simple for this end  user - as I am now left trying guess what the equation is. I think simple would have been to use the Steinhart-Hart polynomial. Many thermistor manufacturers publish the coefficients for this so it would mean that the External Thermistor Model coefficients published in the reference guide could be readily understood. Is there anyway you can escalate this enquiry to see if you can make the equation available?

    From what I have seen on the forum there are quite a few end users who would find this useful.

    Chris

  • 0 in reply to Shirish
    Prodigy 10 points

    Hi Shirish,

    I had another suggestion to resolve my issue if you cannot share the thermistor equation. If I send you some thermistor resistance values, can you tell me the temperatures that would be reported by the BQ34110 based on the default coefficients? I can then compare these with the thermistor datasheet to verify the accuracy of the results.

    Regards

    Chris

  • 0 in reply to Chris Mayo
    TI__Guru 57747 points

    Hello Chris,

    I do not have the data for resistance vs temperature but you can just see the resistance table in the thermistor datasheet and that will be the temperature reported by the gauge.

  • 0 in reply to Shirish
    Prodigy 10 points

    Hi Shirish,

    I think you may have misunderstood. I have started with the thermistor datatsheet values and want to verify the temperature that the BQ34110 gives for each resistance. There is a chart in the BQ datasheet showing error which is significant at low temperatures, so the thermistor datasheet will not be the same as the reported temperature. Obviously if I had the polynomial equation I could find the reported values, but without that I am stuck.

  • 0 in reply to Chris Mayo
    TI__Guru 57747 points

    Hello Chris,

    If you are referring to 6.16 Typical Characteristics, then note that this is for the internal temperature sensor and for the underlying hardware before adjustments/filters are applied.

    See Fig 16 in the datasheet for reported values which is what the gauge uses. (This is with adjustment/filters)

    The external thermistor reported values will be similar or better than the internal sensor reported values.