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BQ78350: BQ78350 charging

Adam
Expert 1225 points
Part Number: BQ78350
Other Parts Discussed in Thread: BQSTUDIO, , BQ76930

Hello,

Im using bq78350 for 5S LiPol battery.

Just have a simple regarding charging.

Charging with 1C current I reach the RSOC 100%. Then shutdown the pack.

But after shutdown command from BQStudio and leaving the battery shutdowned for 15 minutes I've got 86% ROSC when nothing is draining the current.

Two times such situation. I need to check with another batteries but for now maybe someone have any hint for me? Maybe my settings are wrongly?

Regards,

Adam

  • 0
    Expert 1225 points

    by the way.. what does shudtown command actually do. I'm starting with bms topics...

  • 0 in reply to Adam
    TI__Guru* 81035 points

    Hi Adam,

    The BQ78350 uses coulomb counting to count the charge going into and out of the battery to accurately determine RSOC. When a Shutdown command is executed, the BQ78350 sends a command to the BQ769x0 AFE to go into SHIP mode - the BQ769x0 REGOUT goes to 0V (this connects to the VCC of the BQ78350). So the BQ78350 is fully powered down. 

    Coming out of Shutdown, the BQ78350 no longer knows the Remaining Capacity or previous RSOC value. It can only use the cell voltages to make an initial estimate of the RSOC. It uses the ChemID to map the measured voltage to the SOC, so it is important to select the correct ChemID for your battery if you plan to use the Shutdown command frequently. Estimating RSOC based on voltage alone may not be very accurate if the battery is not fully rested.

    I hope this helps,

    Matt

  • 0 in reply to Matt Sunna
    Expert 1225 points

    Thanks, that explains a lot.

    Three more questions:

    Is there any possibility apart voltage way of charge estimation for bq78350? Sth more accurate?

    There is no possibility to associate the Absolute SOC (let say 5000mAh) with certain battery voltage level (e.x. 21V) from BQ Studio settings?

    Letting battery to fully rest provide better accuracy for measureing RSOC after shutdown?

    Regards,

    Adam

  • 0 in reply to Adam
    TI__Guru* 81035 points

    Hi Adam,

    No, unfortunately voltage is the only way for this device since this is the way the firmware is written, and the firmware cannot be modified. 

    Voltage should be fairly accurate if the correct ChemID is selected and if the battery has rested since the last shutdown. Make sure to select the ChemID - the procedure is explained in the Gas Gauging section of the 'Using the BQ78350-R1' app note: http://www.ti.com/lit/an/slua924/slua924.pdf 

    There is also a data flash parameter Remaining Capacity Initial Percent that can be used to scale the initial RSOC if needed.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Expert 1225 points

    Ok, thank you.

    As far as I do not know the chemistry details of the battery I'm forced to do it manually following this instruction: 

    http://www.ti.com/lit/ug/sluub45b/sluub45b.pdf

    I have LiPol 10000mAh 5S battery

    Now have some doubts:

    I'm using physically 0.5mOhm sense resistor instead of the recomendet 1mOhm.

    However settings are used as for 1mOhm.

    CC Gain = 1mOhm

    Capacity Gain = 1mOhm

    Design Capacity 5000mAh

    Learned Full Charge Capacity = 5000mAh

    So in this case should I scale the current in the log file and then use GPC tool?

    Because I'm charging with 10A but the reading is 5A in Bq (which is fine for me).

    Adam

  • 0 in reply to Adam
    Expert 1225 points

    One more..

    where to find Remaining Capacity Initial Percent parameter?

  • 0 in reply to Adam
    TI__Guru* 81035 points

    Sorry, the name of the parameter is RemCap Init Percent. See section 9.1.4 of the BQ78350-R1 Technical Reference Manual.

    To accurately scale the current readings, you need to go through the calibration steps. Use the Calibration screen in BQStudio. The steps for calibration are shown in the bq76930 EVM User Guide.

    Best regards,

    Matt