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BQ24295 : battery charging became slower later

Hank Chu
Expert 1340 points
Other Parts Discussed in Thread: BQ24295

We used the TI BQ24295 charger in the product.

1) We met a battery charging issue on the product as follows:

      We connected the USB cable to the USB port of NB (Windows 7).

      The battery charging became slower later.

1-1) About the details, please refer to the picture below.

2) About the BQ24295

2-1) The Fast Charge Current Limit is set to 1536 mA.

2-2) The Charge Voltage Limit is set to 4.208 V.

2-3) The Thermal Regulation Threshold is set to 120 0C.

2-4) The BQ24295 always displayed Fast & Charging.

 

3) After the voltage is more than 3.39V, the REG08 of BQ24295:

          Bit[7:6] : USB host

          Bit[3] : 1

          Bit[1] :Normal

          Bit[0] : 1

-----------------------------------------------------------------------------------------------------

Could you tell us your comment?

In order to help us debug this issue.

Thank you.

  • 0
    TI__Mastermind 25800 points

    Hey Hank,

    Just to make sure I understand you correctly, the issue you are seeing is that the bq24295 is displaying Fast Charging but it does not seem to be charging at the fast charge rate of 1500mA? So you are seeing a slow increase of the battery voltage. 

    Is this correct? 

    The first thing I notice is that REG08[3] = 1, meaning that the device is in input DPM. This means that the input source (USB port) is being overloaded by any load on Vsys (if any) plus the programmed charge current. The charger is going to reduce the charge current to stay within the Input current limit detected from power up. I would check REG00[2:0] and REG00[6:3] to see what the charger detects as the input current limit and the lower input voltage. 

    And depending on the battery capacity, you may only see such a small change in battery voltage is the charge current is being forced lower. 

    Regards,

    Joel H

  • 0 in reply to Joel Hernandez II
    Expert 1340 points
    Thank you for your kind support.

    1) Yes! It is correct.

    2) About the DPM, we found the descriptions in the Chapter 8.3.2.2 Dynamic Power Management of BQ24295 datasheet.

    3) About the REG00
    3-1) The REG00[2:0] is set to 500 mA.
    3-2) The REG00[6:3] is set to 4.76V.

    4) The battery capacity is 9600 mAh.
    4-1) The charging voltage of battery is 4.2V.

    Q5) Because the system load of product is heavy (input source, USB port of NB, was over-loaded),
    is it NORMAL if the battery charging was getting slow?

    Have a nice day.
  • 0 in reply to Hank Chu
    TI__Mastermind 25800 points
    Hey Hank,

    Because of the input current limit detected, the high battery capacity, and a heavy system load as you say (load on Vsys pin of charger to confirm), then the device, at best is charging the battery at a minimal current (much much less than 500mA) to not overload the bus.

    For example, your USB port can only supply 500mA, and if the load at Vsys is 300mA, that means you only have around 200mA left to charge your battery (some of this is idealized, but conceptually this is what happens). The bq24295 will lower the charge current to avoid crashing the bus.

    In some instances, if your system (Vsys load) is far greater than 500mA, than your battery will not charge, but instead go into Supplement Mode per our device specs, and provide extra current (discharge the battery) to you system.


    So to answer your question, it seems NORMAL that charging is slow.


    Regards,
    Joel H
  • 0 in reply to Joel Hernandez II
    Expert 1340 points
    Thank you very much.

    Now, we understand what you mean.

    Please do us a favor to close this case.