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BQ25730: About Ni-MH charging curve

Part Number: BQ25730
Other Parts Discussed in Thread: BQSTUDIO, , BQ25792

Hi,

Customer is trying to charge a 4cell Ni-MH using a BQ25730EVM and an MCU.
And the charging current will be set to 384mA. Battery is eneloop.

They set the charging voltage to 6V and got the charging curve.
It showed the behavior of increasing the charging current for a moment in the CV charging area.

It is observed that the charging voltage rises a little and the charging current rises during CV charging.
Could you give me some advice on the factors that can cause this behavior?

I want to share charge curve and register information using private messages.
Can you tell me the contact information?

Best regards

Yusuke

  • You can upload your files to this private file folder. 

    tidrive.ext.ti.com/.../8c6fd9dc-bc2d-4a09-b154-8048322ce05d

  • Hi Tiger,

    Thank you for your response.
    I uploaded the charge curve and register setting information.
    Could you give me some advice on why the current rises in the CV charging area?

    Best regards
    Yusuke

  • After reviewing the charge profile and register map, I think the current rises in the CV range is due to MCU command. It looks a new charge voltage is written to the charger as indicated on the charge voltage waveform. Other than this small current rise, the charge profile looks great. 

  • Hi Tiger,

    Thank you for your support.
    >After reviewing the charge profile and register map, I think the current rises in the CV range is due to MCU command.
    I think the charging current has changed due to the increased charging voltage.

    However, customer is using EVM. And Control is done by bqstudio. And I did not change the sent register setting during the test.
    They use bqstudio to fix the charging voltage at the 6V setting.
    Is there a factor that the charging voltage setting changes during CV?
    Is it possible that the charging voltage changes during CV charging due to the characteristics of the battery?

    Best regards
    Yusuke

  • We are talking about 18mV here. Could this be a noise from the battery voltage measurement? It would be interested to know if this observation is repeatable in another charge curve. 

  • Hi Tiger,

    Thank you for your response.
    >Could this be a noise from the battery voltage measurement?
    >It would be interested to know if this observation is repeatable in another charge curve. 

    I have sent you other charge curve information.
    This VBAT rise and current change are reproduced on other charge curves.

    Is there a possibility that the operation of BQ25730 will cause such a voltage rise?
    Device operation information in the CV area is not described in the data sheet.

    And could you give me some advice if there are other waveforms to check?

    Best regards
    Yusuke

  • Another possibility is the NIMH battery characteristics. Note that BQ25730 needs the host to terminate charge. You can monitor the SW node waveform to see if switching frequency changes abruptly. If so, the voltage ripple may change, resulting in a small change on the charge current. 

  • Hi Tiger,

    Thank you for you kind support.
    I got an evaluation board and reproduced the customer's test.
    As a result, the phenomenon was reproduced.

    I will send you the acquired charging curve.
    As an additional test, a 2200uF capacitor was used to check the charging waveform. We will also send you the results.

    Let me ask you three questions about these results.

    1.
    Could you give us your view on the charging curve and the measured waveform?
    From the charging curve, I considered the following:

    ・ Charging current drops in the CV
    ・ Charging current drops sufficiently and pulse charging is executed to check battery connection.
    ・ Ni-MH is recharged by pulse charging and VBAT rises.
    ・ The voltage does not rise at the next pulse charge due to the rise of VBAT.

    Is this understanding correct?

    2.
    How does the BQ25730 behave when the charging current is less than the set value in the CV range?
    Does BQ25730 perform pulse charging to check battery connection?
      
    3.
    Could you explain in detail the control operation of BQ25730 when a capacitor is connected along with the waveform?
    In the case of capacitor connection, it is observed that recharging is repeated.

    Best regards
    Yusuke

  • I think the problem is the current sense resistor. Using 5mohm for low current charge, the charge current resolution is 128mA. We are trying to understand charge current irregularity from 42mA to 53mA during CV phase. The current sense signal level is around 0.25mV. 

    Since the max charge current is 384mA, I would suggest you try 50mohm or higher.

  • Hi Tiger,

    Thank you for your comment.
    If you can figure out the following, please tell me the details.
    And I would like to ask you to explain the operation of BQ25730 for the question 2.
    >We are trying to understand charge current irregularity from 42mA to 53mA during CV phase.


    Recommendation is the understanding of 5mohm or 10mohm.
    Could you give me some advice on the risks of setting 50mohm and the registers to change?
    I don't know how to set the register when applying 50mohm.



    >Since the max charge current is 384mA, I would suggest you try 50mohm or higher.

    Regards
    Yusuke

  • A better part for you, BQ25792: I2C controlled, 5-A, 1-4 cell buck-boost charger with dual-input selector and USB PD 3.0 OTG output

    The minimum charge current is 10mA, compared to 128mA for BQ25730.