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BQ20Zxx limited charge

Kevin McCalmon
Prodigy 130 points

We will be using bq20Zxx fuel gauge. In this particular app, we won’t be charging the cells to full charge, probably 3.9-4.0V. Will there be any issues with using the Z series in less than a full charge capacity. Full capacity would be 4.2 but we’d limit our charge voltage to 4.0.

  • 0
    TI__Genius 10235 points

    No, there won't be an issue with this.  However, you need to alter your learning procedure slightly.  You will need to do 2 learning cycles with 2 different charging voltages. 

    The first learning cycle needs to be done using a 4.2V charging voltage (or whatever is normal for your cell). This allows Qmax and Ra to updated properly.

    The second learning cycle needs to be done using your 3.9V or 4V charging voltage.  This will allow a few other parameters that control what the gauge will report at full charge. This second cycle is necessary so that your gauge will report 100% at your charging voltage.

    Basically, follow the procedure below twice, using 4.2V for charging the first time and 4V charging the second:

    1. Even though accurate capacity is unknown, set Qmax and design capacity to the cell capacity shown

    in the data sheet. Do not send IT enable command 0021.

    2. Send SMB command 46, word 0006 to turn on the FETs.

    3. Discharge the pack down to termination voltage at a C/5 rate.

    4. Let it relax for 5 hours or more.

    5. Send the IT enable command (0021). Bits QEN and VOK in Operation Status are set.

    6. Charge pack to full capacity until the taper current is reached and the FC bit is set.

    7. Let it relax for 2 hours. Qmax is learned at this point; this can be verified by MaxError changing to 3%

    and Update Status to 5%.

    8. Discharge the pack down to terminate voltage at the typical low rate of your application. For laptops, it

    is usually C/5. The MaxError changes to 1%, and the Update Status changes to 6%.

    9. Repeat steps 4 and 6 to 8 one more time to verify the gas-gauging accuracy.