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BQ27621-G1: BQ27621 – SoC Accuracy Issue

Milan Sedlak
Prodigy 10 points

Part Number: BQ27621-G1

Hello,

we are using the BQ27621YZFT-G1A fuel gauge in our product, and we would like to clarify the expected behavior in our specific hardware setup.

System description

  • The Li-ion cell is connected to the BQ27621 through a mechanical on/off switch.

  • When the device is OFF, the switch disconnects the battery completely → the BQ27621 has no supply and no electrical connection to the cell for longer periods (days, weeks, or even months - storage on stock).

  • When the device is turned ON, we:

    1. Power the BQ27621

    2. Run bq27621_init() - configuration update, design capacity update etc.

    3. Wait ~5 seconds for internal stabilization

    4. Read the reported SoC

Observed behavior

After the gauge has been unpowered and disconnected from the cell for a longer period (storage), the first reported SoC after power-up is significantly lower than expected (e.g SoC reported for a new battery is 36% ). Thus new devices reports "empty" battery, even when the battery is nearly full... 

Questions

  1. Is this behavior expected when the BQ27621 is disconnected from the battery for extended periods?

  2. Is there a recommended method to improve SoC accuracy in a system where the fuel gauge cannot remain continuously connected to the battery? (our max storage time should be up to 2 years, therefore we can not keep the BQ27621 permanently connected to the battery)

  3. How long after calling bq27621_init() should we wait before the SoC value can be considered reliable?

  4. Would TI recommend some tips & tricks for systems that must completely power down the fuel gauge while the device is off?

Any guidance would be appreciated.

Best regards,
Milan

  • 0
    TI__Mastermind 39005 points

    Hello,

    This question has been assigned within the team and will be reviewed and followed up with the engineer when possible.

    Thank you,
    Alan 

  • 0
    TI__Expert 7644 points

    Hello Milan,

    1. It is expected to have some gauge inaccuracy after power up the device. The battery will not be same voltage either after a long time. However, once you run a discharge cycle, the gauge should recalibrate again to a good accuracy.
    2. No. Even for shutdown mode, which consumes very little current, and the battery remains connected, we advice customers the gauge will not be able to track SoC during this time.
    3. Usually, it takes a couple of seconds. However, if you send the RESET command, the gauge will be force to take OCV measurement and report a new SoC value.
    4. Please try the following and keep asking for more advice if necceasry:
      1. Try sending the RESET command and let me know if the gauge still reports low SoC when it should be near full
      2. Can you log this and share them with me, so I can take a look?
      3. Do a discharge, and then wait for 60 seconds, and let me know if the gauge is reporting accurately now.

    Was there a voltage drop during the period the battery was disconnected? I imagine if the battery voltage remained the same, then the voltage drop wouldn't have happened.

    It is possible when you disconnect the battery, you are loosing some settings like the Chemid programmed in the gauge. Can you check?

    Regards,

    -Miguel