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BQ24115: How to determine charge done for bq24115?

Harry Li74
Intellectual 470 points
Part Number: BQ24115
Other Parts Discussed in Thread: BQ24105

I bought bq24115 evm and modified it for LiFePO4 batteries and USB, and then I just realised the STAT2 pin is not available for bq24115.

What is the intended way to find out if the charge is done or not?

When the battery reach 3.6V the current do go down to 0, however, STAT1 is always on.

Cheers

Harry

  • 0
    TI__Guru 57651 points
    Harry,

    Before the bq24115 EVM was modified to your application, did the charge current goes down when the battery reach 3.6V and what was the STAT1 behavior?

    Thanks,

    Ning.
    Please click "This Resolved my issue" button if this post answer your question.
  • 0 in reply to Ning T
    Intellectual 470 points

    Hi

    Sorry I did not test the bq24115 EVM before I modified it, because I thought the default 4.2V target may over charge LiFePO4.

    But the only change I made was R5 to 143kohm 1%, the actual voltage stops at 3.575V. Based on documentation "SLUA443–December 2007", "Using the bq24105/25 to Charge LiFePO4 Battery" 

    Cheers

    Harry

  • 0 in reply to Harry Li74
    TI__Guru 57651 points
    Harry,

    When you change R5 to 143kohm, the charge voltage is set to 3.6V. So it is normal that the device stops charging around 3.6V battery voltage.
    The bq24115 is a system-controlled charger and it does not report charge status.

    Thanks,

    Ning.
    Please click "This Resolved my issue" button if this post answer your question.
  • 0 in reply to Ning T
    Intellectual 470 points

    Hi

    So there's no way for my system to know the charge is completed?

    So I just set a maximum timer in my micro controller and disable the charger after the timer? Or just leave the charger always enabled? What is the normal intended design?

    Cheers

    Harry

  • 0 in reply to Harry Li74
    TI__Guru 57651 points
    Harry

    No, the BQ24115 is a system-controlled charger. The host should tell the device when to charge, when to stop charging. If the host is not programmed to control the charger, it is better to use standalone charger such as bq24105.

    Thanks,

    Ning.

    Please click "This Resolved my issue" button if this post answer your question.
  • 0 in reply to Ning T
    Intellectual 470 points

    I understand the BQ24115 is a system-controlled charger. And I do have a micro-controller controlling it.

    How does the system determine if the charge is done or not, if the system have no feed back from the charger? What is the intended system logic to stop charging?

    Cheers

    Harry

  • 0 in reply to Harry Li74
    TI__Guru 57651 points
    Harry,

    The device will terminate charging when the termination current threshold has been reached and will begin a recharge cycle when the battery voltage has dropped below the recharge threshold (VRCG). But the device does not provide an external charge done signal.

    Thanks,

    Ning.
    Please click "This Resolved my issue" button if this post answer your question.