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BQ77915EVM-014: Problem about the output

Jiayi Chen
Prodigy 40 points
Part Number: BQ77915EVM-014
Other Parts Discussed in Thread: BQ76952, BQ77915

Hi guys,

We are using the BQ77915EVM to do some tests.

We connect 3 batteries(3.8V nominal voltage for each) to the BAT terminal, we short the unused pin and also add the shunt on J8. But the output voltage on PACK terminal is still 0V.

Then we connect the power source with 11.4V to the BAT terminal and repeat the test, this time the output on PACK terminal is 10.7V.

Could the cause of this problem be that our battery output is unstable? So in the way, the IC will regard it as the undervoltage or overvoltage.

And if yes, what can we do the solve this problem?

Thanks in advance for your useful answer.

Best regards,

Jiayi

  • 0
    TI__Genius 11625 points

    Hey Jiayi,

    Make sure that the BATT- connection is made first. Then the other cells can be connected in any sequence.

    If your battery voltages were unstable it would more than likely cause an under voltage event. Could you take an oscilloscope shot of BAT+ relative to BAT- when the batteries are connected to see if there is any under voltage or over voltage event?

    Thanks,
    Caleb

  • 0 in reply to Caleb Perez
    Prodigy 40 points

    Hi Caleb,

    Thanks for your reply. I followed what you said

    Caleb Perez said:
    Make sure that the BATT- connection is made first

    but the output of PACK terminal is still zero.

    Here is the osciiloscope shot of our battery set. 

    Would you have some suggestions for us to avoid this protection phenomenon?

    Jiayi

  • 0 in reply to Jiayi Chen
    TI__Genius 11625 points

    Hey Jiayi,

    It looks like there might be an overvoltage event with the ringing taking place. Could you please take oscilloscope readings of the individual cells to see if any one of them might be triggering an event upon connection? 

    Thank you,
    Caleb

  • 0 in reply to Caleb Perez
    Prodigy 40 points

    Hi Caleb,

    I did a lot of individual cell tests on 3 different batteries. The output isn't stable, sometimes it's over voltage, sometimes is under voltage and sometimes is normal. 

    Over voltage cases: 

    Under voltage cases: 

    Normal cases:

    So I would like to ask how can we manage the transcient state of the battery at switch on to avoid overvoltage protection or undervoltage protection?

    Waiting for your reply and thanks in advance.

    Best wishes,

    Jiayi

  • +1 in reply to Jiayi Chen
    TI__Genius 11625 points

    Hey Jiayi,

    I find it very odd/interesting that your batteries are exhibiting such behavior. This is beyond the scope of our device. It will protect accordingly if it senses an undervoltage or overvoltage event. I would try and find new cells to use in your system. 

    If you think the transients are caused by your load, you might benefit from the pre-discharge feature seen on our BQ769x2 family of devices. https://www.ti.com/product/BQ76952 The BQ769x2 family of devices also has customizable UV/OV thresholds among many other things, although it is a bit more complicated to use.

    Thanks,
    Caleb

  • 0 in reply to Caleb Perez
    Prodigy 40 points

    Hey Caleb,

    Thanks for your reply and attention. I will check about the BQ76952 that you recommended. 

    I wish you a good day.

    Best regards,

    Jiayi

  • 0 in reply to Caleb Perez
    Prodigy 40 points

    Hi Caleb,

    I hope you are all well.

    In your opinion, are we supposed to switch from BQ77915 to BQ76952 (because I found all the functions of BQ77915 can be realised by BQ76952) , or just to add the BQ76952 in our system?

    Thanks,

    Jiayi

  • 0 in reply to Jiayi Chen
    TI__Genius 11625 points

    Hey Jiayi, 

    I recommend that you switch from the BQ77915 to the BQ76952.

    Thanks,
    Caleb