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bq20z95 low current accuracy during relaxation mode

Antoine Gardiol
Prodigy 140 points

Hi,

We use bq20z95 in several devices for 3 years and one point stay uncontrolled in spite of all our efforts.

When using the device with currents that put device in charging or discharging mode, all work perfectly.

However, when the device is in relaxation mode for long time with a discharging current of about 380uA, the SOC (watched through remaining Cap) does not follow OCV (watched through bat voltage) as expected (like in Fig 3 of SLUA375 doc) on ~70% of monitored devices (through EV2300 once a day). In fact, voltage fall slightly every day, but remaining cap stays identical (with sometimes a big fall after several days).

About our system :

  • 4 cells battery with chem id 1289 (found with mathcad file provided by TI)
  • 800mAh
  • gg attachedGolden.zip

Thanks by advance for your help.

Antoine

  • 0
    TI__Mastermind 27145 points
    Hi Antione,

    What is the value of your CC Deadband? If the voltage accross your sense resistor is below the CC Deadband value it will not be counted in the algorithm.

    Best Regards,
    Ben Lough
  • 0 in reply to Benjamin Lough
    Prodigy 140 points

    Hello Ben,

    Thanks for your help.

    My CC deadband is 10.0 aka 2.94uV. Since my sense reistor is 27mR, this value indicates that current below 109uA will not be counted. My average current is ~380uA, so it should be counted, no ?

    On the other end, maybe my comprehension is not good, but I understood that the SOC should follow the OCV based on chemistry profil in relaxation mode since Coulomb counter is not precise enough for low current. Is it correct or not ?

    Thanks by advance.

    Antoine

  • 0 in reply to Antoine Gardiol
    TI__Mastermind 27145 points

    Hi Antoine,

    Do you have some data I can look at?

    Best Regards,

    Ben Lough

  • 0 in reply to Benjamin Lough
    Prodigy 140 points

    Hello Ben,

    What kind of data do you want ?

    I have the following excel file that show 8 devices logged once a day with bqEasy software. 3 of them works as expected and 4 have a "flat" behaviour. The gray lines of graph have been logged on other devices with a greater power cunsumption.

    LogBatTI.zip

    Please note that each device is put in sleep mode and waked (by SMBus com) only each ten minutes by uc to read RSOC.

    Can you also comment my previous post and my understanding of bq20z behaviour ?

    Thanks by advance.

    Antoine

  • 0 in reply to Antoine Gardiol
    Prodigy 140 points

    Hi Ben,

    I just realized that the CC deadband shown is the GG file is not the raw value from the flash but a converted one in uV unit. So my 10.0 does not mean 109uA but 370uA. That value is very close to my real current (~380uA) and your idea seems a very good tip so.

    I changed now the CC deadband to 5.0 and I will see next Monday if it solves my issue.

    In the meantime, can you confirm that the SOC is computed only on the CC in relaxation mode and OCV is used only with RaTables when not in relaxation mode ?

    Thanks by advance.

    Antoine

  • 0 in reply to Antoine Gardiol
    TI__Mastermind 27145 points
    Antoine,

    OCV is still used in relaxation mode. Since your discharge current is small, your battery voltage will be close to the actual open circuit voltage. As long as your CC deadband is set appropriately, the coulomb counter will also contribute to the SOC calculation in relaxation mode.

    Best Regards,
    Ben Lough
  • 0 in reply to Benjamin Lough
    Prodigy 140 points

    Hello Ben,

    Thanks for your explanation.

    Unfortunately, after several days of test, my problem is not solved by the change of CC deadband (see this file LogBat_with_Golden_281116.zip).

    The deadband is set at 2.9uV (means ~107uA with my 27mR sense resistor) since saturday and only one of the 8 devices has counted correctly the current flow. In fact the behavior is nearly the same as before.

    Any other idea ?

    Thanks by advance and best regards.

    Antoine

  • 0 in reply to Antoine Gardiol
    Prodigy 140 points

    Hi Ben,

    After several weeks of documentation reading and test, we have identified the cause of our problem : when no sleep mode is used, Remaining Cap is updated every 5 hours in relaxation mode. When we activate the sleep mode (Sleep Voltage Time is 5s and Sleep Current Time is 20s), remaining capacity is not well estimated and the bit R_DIS in Operation Status Register stay at 1 instead of falling down 30 minutes after entering relaxation.

    Conclusion : in sleep mode, it looks like the chip cannot detect or stay in relaxation mode !

    Do you have any information about this behavior ?

    Thanks by advance and best regards.

    Antoine