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BQ77915: BQ24105 + BQ77915 issue

Majid HAMAOUI
Prodigy 10 points
Part Number: BQ77915
Other Parts Discussed in Thread: BQ24105

Tool/software:

Hi,

I’m having a problem with the BQ77915 and the BQ24105 charger.

When the batteries are fully discharged, the system won’t start charging again.

The output of the BQ24105 shows 3.13V, and the MOSFETs of the BQ77915 are OFF.

If I disconnect my LOAD from VBAT, the voltage goes up to 9.2V, which matches the 3-cell VBAT.

Here is my schematic. Do you have any idea what might be wrong?

Thanks!

  • 0
    Prodigy 10 points

    One clarification: the LOAD is connected to the PACK+ and PACK- terminals.

    When the batteries are in undervoltage (fully discharged), and I apply 14V to VCC, nothing happens, and the current stays around 50mA (I changed RISET2 to 20kΩ).

    If I disconnect the LOAD (ESP32 + LCD...) from PACK+/-, the current increases to 1A.

    I think the BQ24105 detects an overcurrent and limits the voltage.

    So how to resolve this issue.

  • +1
    Prodigy 10 points

    The problem was solved using two diodes:

    1. First diode (VCC to LOAD):

      • This diode powers the LOAD (ESP32, LCD, etc.) directly from the external power supply (VCC) when the batteries are in undervoltage or precharge mode.
      • It prevents the LOAD from overloading the battery system during this time.

    2. Second diode (PACK+ to LOAD):

      • This diode allows the LOAD to switch automatically to the battery (PACK+) once the batteries are sufficiently charged.
      • It also prevents any current from flowing back into the BQ24105.

    This setup ensures the LOAD is always powered without triggering the BQ24105’s overcurrent protection. 

    What do you think? Any comment?

  • 0 in reply to Majid HAMAOUI
    TI__Guru**** 152856 points

    Hi Majid,

    That sounds reasonable.  

    Regards,

    Jeff