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BQ25172: 2S2P NiMH charging for 1 hour and then stop charging

Tom Hong
Prodigy 40 points
Part Number: BQ25172

Tool/software:

My project uses 2S2P NiMH batteries(900mAh).

Image waveform: STAT When there is no battery, I want to know if this waveform is wrong?

During the test, the battery stopped charging after about an hour, and the battery voltage was measured at 1.34V~1.35V. The battery is discharged to obtain a power of 220mAH~280mAH.
The battery will be discharged to 0.8V before testing.

It is set to fully charge in 6 hours, I don't understand why it stops charging after 1 hour?

  • 0
    TI__Guru* 75876 points

    Hi Tom

    Tom Hong said:
    Image waveform: STAT When there is no battery, I want to know if this waveform is wrong?

    This may be OK, in a no-battery condition the output voltage will overshoot set BAT OVP turn off and battery voltage drop then cycle restarts.  Check the output voltage and see if it is reaching 3.4V (1.7V X 2)?

    Tom Hong said:
    During the test, the battery stopped charging after about an hour, and the battery voltage was measured at 1.34V~1.35V

    The schematic looks okay.

    One hour timer setting does not match any configuration.

    If the battery voltage increases to 3.4V it would trigger OVP but STAT LED would BLINK at 1 Hz.

    Do you have current meter in the output? Resistance of shut has caused problems in the past.

    You are using a signal ground plane and power ground plane, could be a factor.  

  • 0 in reply to Bill Johns
    Prodigy 40 points
    Image waveform: STAT When there is no battery, I want to know if this waveform is wrong?
    This may be OK, in a no-battery condition the output voltage will overshoot set BAT OVP turn off and battery voltage drop then cycle restarts.  Check the output voltage and see if it is reaching 3.4V (1.7V X 2)?
    Add OUT(pin8) and ISET(pin2) as shown in the picture, in the state without battery.
    The OUT voltage max will not exceed 3.6V and will not trigger the over voltage protection (1.7V X 2).

    Do you have current meter in the output? Resistance of shut has caused problems in the past.

    The current meter was not used during the test.

    The following will continue to add waveforms measured by the oscilloscope.

    .

  • 0 in reply to Tom Hong
    TI__Guru* 75876 points

    Hi Tom

    Thanks for the additional information and scope capture.

    No Battery Photo -- We can see the OUT increase above OVP, 3.4V and charge stops.  The output capacitor is charged and will discharge down to recharge point and charge restarts.  Battery recharge point is 1.33V X 2 or 2.66V.  This is looking correct.  The OVP and recharge is indicating unit is set to 2 cells as expected.

    Battery Charge Photo -- The ISET pin voltage is about 1V which is correct, not low.  This would indicate that the unit is regulating to target current set by the ISET Resistor.  But you are how low of target current in this test condition?

  • 0 in reply to Bill Johns
    Prodigy 40 points

    The following two photos show the voltage and current values ​​at the input end.
    However, the charging IC (Bq25172) automatically stopped charging after 1 hour and 10 minutes.

    I tried to cancel the intermittent charge function, and changed VSET to 36Kohm. The charging time can reach 5 hours and 53 minutes.

    Don't understand why using intermittent charge will cause charging to stop early?

  • 0 in reply to Tom Hong
    TI__Guru* 75876 points

    Hi Tom

    I would like to work with you on this problem using the Private Messager feature of the forum.  I have sent you a request after accepting I can set up a conversation with you.