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BQ35100: SOH Merit in EOS Mode

corey
Prodigy 110 points
Part Number: BQ35100
Other Parts Discussed in Thread: EV2400, BQSTUDIO

There are various posts on this topic such as https://e2e.ti.com/support/power-management/f/196/p/963480/3563310#3563310, but it's still not clear what the proper sequence is for proper EOS mode operation without causing the SOH_MERIT bit to be set. In my host MCU I tried the sequence posted by Wyatt Keller in that post but I still see the SOH_MERIT flag even with a fresh battery. I can reproduce the same behavior using the bq35100EVM connected to my PC with an EV2400. These are the steps I'm using to reproduce with the bq35100EVM.

Setup: Configure jumpers for 2-4s on J2 and 2S on J3. Calibrate using bqStudio. Set appropriate configuration using bqStudio, for example, program chemistry ID (609 in my case as I'm testing with Tadiran TL-5920 cells), set cell count = 2 since I'm using 2 series cells, set capacity (8500 mAh), enter EOS mode by setting GMSEL1=1. Issue NEW_BATTERY command and attach new battery.

1. Cycle gauge enable using GE PULL_UP jumper.

2. Issue GAUGE_START command.

3. Apply load between BAT+ and PACK- pins of J6 and wait some time, say ~60 seconds. Currently I'm using a 60 Ohm resistor for a load. This ensures that the voltage drops by more than the recommended 100 mV, in fact it drops by about 800 mV.

4. Remove the load.

5. Issue GAUGE_STOP command.

6. Wait at least 15 seconds to allow for gauge to complete processing and write values to flash.

7. From "Registers" tab click refresh and observe that SOH_MERIT is set.

So here are my questions.

1. Can you suggest a sequence that will result in a proper EOS detection cycle using the bq35100EVM that will not result in the SOH_MERIT bit being set?

2. When is the appropriate time to check the SOH_MERIT bit? Currently I am always reading it after the GAUGE_STOP command has completed. That is, after issuing the stop command I wait until GA (gauge active) bit is low and G_DONE bit is high before checking SOH_MERIT. However, if I check the SOH_MERIT bit before the GAUGE_STOP command I noticed that it is not set. So when should it be considered valid?

Thanks.

  • 0
    TI__Guru** 105760 points

    Hello Corey,

    The SOH_MERIT does not relate directly to the EOS mode, the EOS detection will still work with the flag set.

    The SOH data in EOS mode is not intended to be as accurate, the main use for EOS mode is for End Of Service detection using the alert.

    Following the procedure outlined in the thread linked should lead to good resistance updates.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 110 points

    Well, my end goal is to have a relatively accurate SOH, but since we're using Lithium Thionyl Chloride primary cells I was led to believe that EOS mode was the appropriate mode since the technical reference manual states that the SOH mode, "is suitable for determining SOH for Lithium Manganese Dioxide (LiMnO2) chemistry."

    I also see the note that states, "In EOS mode, the accuracy of the SOH reported value can vary significantly with a load profile. Perform in-system evaluation to determine the reported value at the desired EOS level. In some instances, the value of SOH should be ignored."

    So if EOS mode cannot provide reliable SOH data can SOH mode be used for LiSOCl2 cells, or is there just no way to obtain a reliable SOH reading when using such cells using this gauge?

    Thanks.

     

  • 0 in reply to corey
    TI__Guru** 105760 points

    Hello Corey,

    Getting a accurate SOH for LTC battery chemistries is very hard while using minimal power to measure the battery.

    SOH mode is designed for LiMnO2 chemistry only, LTC battery chemistry is too flat to use this method of gauging. The only way we can get some SOH data in EOS mode is by correlating resistance values from the chem ID, but it is not designed for this functionality, the main purpose is just for EOS detection.

    You can try performing tests as stated in the TRM to see if this is reliable in your system.

    You can perform a learning load pulse that is always constant current that will give the gauge a consistent resistance measurement every time, instead of relying on your system load.

    Sincerely,

    Wyatt Keller