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BQ34Z100 SoH calculation

Base Alfa
Prodigy 30 points
Other Parts Discussed in Thread: BQ34Z100, BQ34Z100-G1

I'm looking to a solution to determine the State of Healt of Acid-Lead batteries (the State of Charge is not that important).

I understood the BQ34Z100 can provide some indication on this.

How is (in terms of reliability) the indication from this chip vs EIS (Electrochemical Impedance Spectroscopy) type of analysis 9like the one mentioned in http://www.ti.com.cn/cn/lit/wp/spry215/spry215.pdf) ?

The disadvantage i see on the BQ34Z100 is that I need to put a shunt in the circuit and (but i'm still trying to understand how the Impedance Track actually works) be able to take measurements in almost Open Circuit conditions (Relaxation period), which i'm not sure I can do.

Or do I understand wrongly the way the BQ34Z100 works?


thanks for any comment / clarification

  • 0
    TI__Guru 73830 points

    You can find an overview of IT in our Application Book. Search for SLUA404 on the TI website. It was written for a different device, but the concepts are the same. IT does require rest periods to allow the cell voltages to stabilize to take an OCV reading to estimate capacity. It also uses a Ra table to simulate the DC series resistance in the cell and the simulation has transient modeling to compensate for transient resistance in the cell. If you cannot allow rest periods in your application, then you will need to consider one of our CEDV gauging algorithm devices.

  • 0 in reply to ThomasCosby
    Prodigy 30 points

    Thanks Thomas for your feedback.

    I'm reading the documentation and eventually looking into the CEDV solution.

    Still on the Impedance Track approach: will it work even if the rest periods are normally occurring when the battery is fully (or almost) fully charged?

    As for a typical BBU, in normal operation the power is not provided by the battery but from an external source. The battery is there just in case. Sometime the load and the charger are turned off, so eventually i could use these rest periods to do some measurement, but in most of the cases these will be at an almost fully charged status for the battery. Will be these readings OK for the I.T. to determine the Healthy status of the battery?


    thanks again

  • 0 in reply to Base Alfa
    TI__Guru 73830 points

    SOC and SOH accuracy will be dependent on how well Qmax and the Ra-table match the cells. Learning will need to occur as the used and as it ages. The device updates these parameters as the battery is used and allowed to rest. You will need to at least partially discharge the pack periodically (maybe every 1 to 2 months) to update the Ra-table and Qmax. The pack will need to rest after the discharge period to update Qmax. If learning's allowed, then the device should be able to provide good SOH estimates. 

  • 0 in reply to ThomasCosby
    Prodigy 30 points

    Thanks again Thomas for your clarifications.


    I'm not sure a partial discharge will be possible; or - better - the battery will certainly be partially discharged during the normal use, but immediately charged back. Which means that the moment the battery will be on a steady state and ready for a new measurement it will be in a (almost) fully charged state. But i will investigate this further and eventually come back to you for additional clarifications

    thanks

  • 0 in reply to ThomasCosby
    Prodigy 130 points

    Hello,

    We are trying to get SoH updates on our battery-packs. Learning cycles were already made and even an optimization process from your part. The thing is that we cannot see SoH updates during our burning and accelereted tests.

    I can see in this post that Qmax updates need resting periods and found a section talking about this issue on the device (bq34z100-g1) datasheet, section 8.3.6.8 Quit Current, Dsg Relax Time, Chg Relax Time, and Quit Relax Time.

    The fact is that we accomplish all the requirements for Qmax updates (at the moment quit current=40mA, Dsg Relax Time=60 sec, Chg Realx Time=60 sec), as we periodically charge the battery-pack to SoC=100% and leave it without draining it for 7 minutes. The electronics consume 17mA from the pack, so I guess that as it is a lower value than the Quit current, it should work.

    Any recommendations?

  • 0 in reply to IKER JOARISTI URTUSAGASTI
    Expert 2475 points
    The gauge has to have DOD0 updates to allow Qmax to update. The pack should be allowed to rest until the OCVTAKEN flag sets indicating that a DOD0 point has set and this will take more than 7 minutes of rest. If you can attach a log file showing your charge and discharge cycle and a gg file from the pack, then we can review them. You also mentioned that you are running burn-in testing. Please note that Qmax will not update if the temperature is higher than 40 degC.