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BQ35100: Gas Gauging without forced discharge of battery? Calibration?

Yu Fei Leung
Prodigy 120 points
Part Number: BQ35100
Other Parts Discussed in Thread: EV2400,

We're using an EV2400 evaluation kit.  We use a battery pack of 2 Li-SOCl2 batteries in parallel, 3.6V. Our application normally draws 30μA, up to about 90μA when idle, with higher spikes of up to 10mA during activity.

We had some questions about Gas Gauging and Calibration.

Regarding Gas Gauging:

  • I see in the datasheet  on page#10 that it mentions a "Battery Fuel Gauge" (as distinct from the "End-Of-Service Monitor" ), and says that this works to "provides gas gauging for lithium thionyl chloride (Li-SOCl2) and lithium manganese dioxide (Li-MnO2) primary batteries without requiring any forced discharge of the battery ." (emphasis mine)
    • The datasheet mentions that forced discharge of the battery is not necessary. Is that true for all modes? It seems like EOS mode requires us to create a high load pulse (10's of mA for 10's of milliseconds)?  That seems to me to mean a minimum of 20mA for 20ms.  We would actually prefer it if we did not need to perform this kind of forced discharge of the battery!
    • If indeed that high load pulse is necessary for EOS mode, but the maximum we can generate is only 10mA (and not 20mA), how much accuracy would we lose in our EOS measurements?

Regarding Calibration:

  • Is calibration supposed to be done for each and every instance of a battery pack + main-board (and thus needs to also be re-done when batteries are replaced on an existing board)?  Or, does this only need to be done once for a particular battery model and battery pack configuration, after which the settings derived from that could be applied universally for every instance of a given main-board design?
  • If the calibration is required, what calibration (Voltage, cc offset, board offset, current, temperature) do we have to do for each operation mode (ACCU, SOH, EOS).
  • If current calibration is required, how could we do it?  How (when) is a “Known load” is applied to board?

Finally, we were also wondering about SOH (State-Of-Health) mode.  We know that the datasheet says it's meant to be used for lithium manganese dioxide (Li-MnO2) chemistry, but would it still be possible to use SOH mode with lithium-thionyl chloride (Li-SOCl2) chemistry?  Even if perhaps it may have reduced accuracy, could it perhaps still be used, and if so, what would be the loss of accuracy involved?

Thanks!

Regards,
Yu Fei Leung

  • 0
    TI__Guru** 105760 points

    Hello,

    The gauge only requires these conditions during the discharge pulse, the discharge is not a forced discharge because the load should be from your systems normal operation.

    You will need to generate a 100mV drop across the batteries for accurate gauging of the pulses.

    The calibration would need to be done for maximum accuracy, I would refer you to chapter 3 in the TRM for calibration information and when it is needed.

    Using the SOH is possible in EOS mode, but the accuracy is really dependent upon your application and the ChemID used. Other E2Es have discussed this: https://e2e.ti.com/support/power-management/f/196/t/799572?BQ35100-Problems-reading-SOH-in-EOS-mode#pi320995=1

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 120 points

    Wyatt Keller said:

    The calibration would need to be done for maximum accuracy, I would refer you to chapter 3 in the TRM for calibration information and when it is needed.

    I see.  Am I correct in understanding that the calibration needs to be done only once for each BQ35100's lifetime?  In other words, done only once at the factory, and that calibration does not need to be re-done when new batteries are inserted?

    Wyatt Keller said:

    Using the SOH is possible in EOS mode, but the accuracy is really dependent upon your application and the ChemID used. Other E2Es have discussed this: https://e2e.ti.com/support/power-management/f/196/t/799572?BQ35100-Problems-reading-SOH-in-EOS-mode#pi320995=1

    Sincerely,

    Wyatt Keller

    Thanks for linking to that thread, it's very informative.  It seems it might be tricky to get SOH data.  For us, it would be a "nice to have", but not strictly necessary.  As long as we can obtain reliable EOS data, it will meet the needs of our application.

  • 0 in reply to Yu Fei Leung
    TI__Guru** 105760 points

    Hello,

    Yes that's correct, the calibration should be performed in the factory only once.

    Then I would say you could pull the SOH data during testing and see if it is accurate enough to use for your application.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 120 points

    Hi Wyatt,

    Thank you!  We were wondering if TI offers a factory-calibration service, something where TI (or a distributor of yours) will calibrate the BQ35100 chips first before shipping them to us?  And if so, what would the added cost be?

    If TI does not offer such a service, then do you have a calibration jig (with a socket to insert BQ35100 chips) that we could purchase in order for us to perform this calibration?  The reason we ask is because our production board is unfortunately not designed in such a way as to facilitate calibration operations (such as supplying a known 3.6V input or pulling a known current load).

    If there's no such calibration jig available for purchase, then perhaps you could advise us on how we could build our own calibration board with socket?

    Thanks!

    Regards,
    Yu Fei

  • 0 in reply to Yu Fei Leung
    TI__Guru** 105760 points

    Hello,

    We do not offer this service because some of the calibration is based on your PCB layout.

    Once the calibration is done on your PCB the calibration data is saved as well and the whole gauges configuration can be exported and uploaded to your gauges.

    In the EVM users guide the data exported in the file is discussed in section 3.2 and the calibration procedure is section 3.3: https://www.ti.com/lit/ug/sluubh7/sluubh7.pdf?ts=1594417507537&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FBQ35100

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 120 points

    Hi Wyatt,

    OK, I understand that the calibration is based on PCB design (so the calibration settings could vary from one board design to another).  Do I understand you correctly that the calibration would only need to be performed on a single gauge on one instance of our PCB, and then we could export those calibration settings to all of our gauges that will be used on the same PCB design?

    Also, do you have an estimate of the error rates if we do not perform the calibrations?  We're especially wondering if we would still be able to get End-Of-Service reliably, for example.

    Thanks!

    Regards,
    Yu Fei

  • 0 in reply to Yu Fei Leung
    TI__Guru** 105760 points

    Hello,

    You can do the calibration on a few of your boards and then average the calibration values for the best result. Take the averaged values and use them in your golden programming file for the rest of the boards.

    The gauges should be able to accurately determine EOS using the method described, I am not sure of the exact error because it depends on your application.

    Sincerely,

    Wyatt Keller