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BQ2970: Battery Pack needs momentary manual bypass of Mosfets for functioning in some of the boards

Intellectual 465 points

Replies: 6

Views: 138

Part Number: BQ2970

Hi,

I am testing a recently designed Lithium Ion  Battery Pack Board with BQ24296 Charger, BQ27542 Fuel Gauge and BQ29701 protection IC.

In board/schematic ALL grounds are connected to CELL-. 

In some of the boards , when I power up first time, voltage across Pack+, Pack- is  zero (circuit is open) though have a fully charged battery. Once I short protection IC Mosfets momentarily , get voltages at Pack+,  Pack- and it works normally afterwards. Protection IC works fine later and trips of OCD, UVV etc.  

I hope this is not an expected behavior. Please suggest what could be wrong and how to take care of it.

Part of schematic is below. Would be great if you can review the schematic too. Will directly mail you. 

Thanks,

Anil

  • Hi Anil,

    Thanks for reaching out. I have a few questions about your setup. When you first power on the device and PACK+ and PACK- are an open circuit, what is the status of the rest of the pins? I just want to make sure the device is not triggering some sort of fault. Specifically, can you check to make sure:

    • Make sure the FETs are on. This is the biggest indicator that a fault may be occurring.
    • The difference between the BAT and VSS pins is above 2.3V and below 4.28V? These are the UVP and OVP for your device.
    • Check the voltage between V- and VSS. This is used to indicate any current protections.

    Once you have verified that the device is not in any fault mode, I would like you to try shorting the V- pin to the VSS pin. On page 17 in our datasheet, we state that the circuit may not be enabled when the battery is first connected. Shorting these pins should fix this issue. Alternatively, you could also connect a charger to recover.

    Thanks,

    Shawn

    TI provides design resources and design advice (including technical, schematic, code and/or system review) “as is” and with all faults.  All information in this or related correspondence is provided subject to TI’s Important Notice (http://www.ti.com/corp/docs/legal/important-notice.shtml).

  • In reply to Shawn Hinkle:

    Thank you very much,  Shawn for the quick answer.

    Yes,  Pack is in no fault state - Cell voltage is  around 3.7V , within UV, OV trip levels and no load at P+ and P-

    Shorting V- and VSS  momentarily  fixed the issue. 

    Sorry for posting a clarification which is already available in datasheet  !!!

    Since shorting V- and VSS is  not practical in production,  would like to try other recommendation - connecting charger between Pack+ and pack-. What should be the voltage and current limit for the charger . Is 5V/2A  OK ?

    Btw, will using a 100 nF cap across V- and VSS will help for this initial startup issue ?

    Could you also comment on connecting all the 3 ICs (charger, Fuel Gauge, Protection IC) ground to a common point - Cell- ?. Is it OK ?. Can I mail you the schematic  and get it reviewed. Will definitely be helpful since I will be releasing  the board to production soon.

    Also, would like to know when  BQ29728 and BQ29731  be available in production. These devices' spec  matches exactly with my requirements.

    Thanks & Regards,

    Anil

  • In reply to Anil Sudhakaran:

    Hi Anil,

    As long as the pins connected to PACK+ and PACK- (which are BAT and V- respectively) are still operating within the recommended range on the datasheet, it should be fine. I see that the the voltage for that pin has an absolute max of 12V, so a 5V charger should be okay. 

    I'm not sure if adding the cap will solve the issue, but I think it would be interesting to try. Let me know if you decide to do this and what the results are. 

    If you would like me to review your schematic privately, you can send me a friend request and a message. 

    If you are interested in ordering one of those devices, please contact your local sales representative. 

    Thanks,

    Shawn

    TI provides design resources and design advice (including technical, schematic, code and/or system review) “as is” and with all faults.  All information in this or related correspondence is provided subject to TI’s Important Notice (http://www.ti.com/corp/docs/legal/important-notice.shtml).

  • In reply to Shawn Hinkle:

    Hi Shawn,

    Connecting 5V power to Pack+ and Pack-  momentarily worked.  Thanks a lot for your  help.

    Connecting  a 47 nF cap across the MOSFETS seems to  work. Tested on one board. It worked. Will try on more  boards.

    Please accept my connect request so that I can send  my full schematic to you for review. 

    Thanks & Regards,

    Anil

  • In reply to Anil Sudhakaran:

    Hi Anil,

    I'm glad those actions worked! I have accepted your friend request so we can continue the schematic review over direct message.

    Thanks,

    Shawn

    TI provides design resources and design advice (including technical, schematic, code and/or system review) “as is” and with all faults.  All information in this or related correspondence is provided subject to TI’s Important Notice (http://www.ti.com/corp/docs/legal/important-notice.shtml).

  • In reply to Shawn Hinkle:

    Thanks Shawn.

    "Resolving" this discussion thread. 

    Regards,

    Anil

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