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BQ27742-G1: How close CHG FET after Full Charge Termination Detection

Asael7
Intellectual 270 points
Part Number: BQ27742-G1

Hi.

When full charge termination is detected, CHG FET is opened because I set to  1 the FCFET in Pack Configuration D.

Once I remove the charge I want the BQ close the CHG FET, but that, doesn't happen.

I understood that this behavior is related to the parameters FC Set %, FC Clear %, TCA Set % and TCA Clear %. The configuration that I have for those parameters are, FC Set % = -1, FC Clear % = 98, TCA Set % = -1, TCA Clear % = 98.

The problem with not close the CHG FET when is fully charged is that I have 4.2 V in the battery but after the CHG and DSG FETs, I have 3.5~3.7 V (CHG FET is open and DSG FET is close), the voltage drop is caused by the forward diode voltage of the CHG Mosfet.

So, after a discharge and when the state of charge is less to 98%, CHG FET is closed.

I have some questions:

1. If I don't modify the configuration of the parameters mention above, how could the BQ close the CHG FET once the charge is removed? Maybe considering a minimum discharge, ¿What parameters should I configure? .

2. If I modify the configuration of  the parameters mention above, how should I configure the parameters to close the CHG FET once the charge is removed?

3. If the activation of the CHG FET is not related with the parameters mention above, what paramaters are related?, and how I should configure them to close the CHG FET once the charge is removed?

  • 0
    TI__Guru* 76230 points
    Hi Martin
    Pls review section 2.6.4. of the technical reference manual. For the charge fet to stop charging the FCfet flag in pack config D register has to be set.
    For the FC flag to get set, you need to configure your taper current appropriately. The taper current programmed in data flash has to be greater than the taper of your charger to ensure that FC flag gets set.
    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Intellectual 270 points

    Hi Onyx.

    The stop charging works well. But maybe there was a misunderstanding.

    What I want is after the charge is stop, and I remove the charge, I want the CHG FET close , but doesn't happen, it remains open.  The CHG FET close until there is a discharge and the state of charge is 98%.

    How can I get the FET to close once the charge is removed without having to discharge (without apply a load) and without reaching a  state of charge below 100%?

  • 0 in reply to Asael7
    TI__Guru* 76230 points
    Hi Martin,
    Why would you want the charge fet to close when there is no discharge applied. There is no way around this except you disable the charge fet from opening when charge completes.
    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Intellectual 270 points
    Ok, I understand that is no way to close the CHG FET if a discharge is not applied.

    Then, once the charge is stop, if I removed the charge and I apply a load, the only way to close the CHG FET immediately after applying the load is configure the TCA Clear % and/or FC Clear % at 99% or even 100%? or is there another way?
  • 0 in reply to Asael7
    TI__Guru* 76230 points
    The fet closing is not dependent on those flags. Once you apply a load, the body diode of the charge fet starts to conduct and in order to prevent the diode from getting damaged, the charge fet will come on.
    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Intellectual 270 points
    Thanks Onyx. The problem was solved.