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BQ76925: Please help to suggest a monitor that supports 4 Primary battery cells

Leo Yang
Expert 5735 points
Part Number: BQ76925
Other Parts Discussed in Thread: BQ76905

Tool/software:

Hi team,

My customer wants to have a battery monitor to monitor every cell's voltage. And the battery is Primary battery.

Cylindrical Primary Lithium 1.5V for a battery.

Could you help to check BQ76925 can do that? We checked BQ76905 before and realized it's VC needs to be greater than 2V. And the battery is only 1.5V per cell.

Please help to check.

Thanks,

Leo

  • 0
    Prodigy 90 points

    Hi, Sir,

    But the BQ76925 Vbat min is 4.2V, our design now is Cylindrical Primary Lithium*4. (so the voltage range is 3.2~7.32V).

    and we want the Vbat range at least 3V.(same as BQ76905).

    also we want to monitor each battery voltage and total current consumption.

    Thanks!

  • 0 in reply to stanley wang
    TI__Genius 17875 points

    Hello Stanley and Leo,

    BQ76905 should be the best fit from what you all have shared. There might be some confusion in the minimum cell input voltages. If you were using VC5, you would need at least 2V common mode, however, VC1 to VC4 can go down to 0V common mode.

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Prodigy 90 points

    Hi Alexis,

    but from datasheet, that show the connection about 4 cells,

    and accroading to your description, can we use VC1 to VC4?

    if so, can you share the connection/design?

    Thanks!

  • 0 in reply to stanley wang
    TI__Genius 17875 points

    Hello Stanley,

    Apologies for the confusion. I will reword my previous reply. VC5 needs at least 2V common mode. Please follow the table in shorting the connections.

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Prodigy 90 points

    Hi, Alexis,

    if so, maybe for our application, we can't select BQ76905?

    do you have other solution to meet our requirement? maybe must use discrete solulation. (ex. one chip to get voltage + the other one to get current)

    Thanks!

  • 0 in reply to stanley wang
    TI__Expert 5735 points

    Hi Alexis,

    Could you help to find one suitable for Stanley?

    Thanks,

    Leo

  • 0 in reply to Leo Yang
    TI__Genius 17875 points

    Hello Stanley and Leo,

    I think there is still some confusion. BQ76905 should still work. VC5 needs 2V common mode. This means, even if the differential of VC5-VC4A is <2V, as long as VC4A to VSS is >2V it should be fine. The resolution may not be as good.

    If the voltage range of the 4S cylindrical primary lithium is 3.2V, a minimum of each cell should see 3.2/4=0.8V, 3 cells below top cell at minimum cell voltage --> 3*0.8 = 2.4V --> 2.4 > 2V common node.
    As shown in the datasheet, the minimum cell input voltage is the MAX (VCx-0.2 OR VSS-0.2).



    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Prodigy 90 points

    Hi Alexis,

    Got it, BQ76905 should still work. is there any information about the accuracy? if VC5-VC4A is <2V...

  • 0 in reply to stanley wang
    TI__Genius 17875 points

    Hello Stanley,

    The cell voltage measurement accuracy and stack voltage measurement accuracy are shared below and can be found in Section 6.12 Analog-to-Digital Converter (continued) of the datasheet.



    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Prodigy 90 points

    Hi Alexis,

    I'm confused, from the information you provide, that means even the condition "VC5-VC4A is <2V, as long as VC4A to VSS is >2V it should be fine. The accuracy may not be as good."

    even so, the accuracy is also at -3.8~3.9mV?

  • 0 in reply to stanley wang
    TI__Genius 17875 points

    Hi Stanley,

    Apologies, I mistakenly used accuracy when I should've said resolution. I will correct the previous post.

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Prodigy 90 points

    Hi Alexis,

    okay, but the same question, just want to know is there any data or information from datasheet about the resolution when VC5-VC4A is <2V and >2V?

    if not, when we get HDK, we just try to verify BQ76905 function is work or not.

    thanks!

  • 0 in reply to stanley wang
    TI__Genius 17875 points

    Hello Stanley,

    Information on the resolution can be found in Section 6.12 Analog-to-Digital Converter from the datasheet. It won't say the resolution may not be as good when VC5-VC4A is <2V. It is a possibility as the batteries being used are only using a small portion of the entire ADC range our device can provide for. 
     
    Best Regards,
    Alexis