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BQ25120: When does it switch to CV loop?

Vala
Expert 1110 points
Part Number: BQ25120

Hi,

I am using BQ25120 as the power management core of my device. I was under this impression that it switches from constant current to constant voltage loop when the battery voltage hits the regulated battery voltage, which in my case is 4.1V. But I noticed that the current starts to drop from the value set in FAST_CHARGE_CONTROL register (in my case 60mA) when the voltage is significantly lower than 4.1V e.g. about 3.9V. How does it work? I set the INLIM current to 250mA and disabled the timer. I also monitored the status register, there was no fault, VINDPM_STAT was zero all the time and it was showing that charging is in progress. For example the graphs in this tutorial show that the CV profile takes the control when the battery voltage is equal to battery regulated voltage.

Another question regarding this issue. If changing from CC to CV depends on the battery voltage, how this battery voltage is sensed by the chip? Is it the output from VOLTAGE_BASED_BATTERY_MONITOR register (address 0x0A)? Is it measured with charger connected?

Vala

  • 0 in reply to Vala
    TI__Intellectual 2050 points
    Vala,

    I am wondering if Dynamic Power Path Management(DPPM) is having an affect, or possibly a thermal situation.

    What is your input voltage? Do you have a load connected on PMID and/or SYS?

    Are you using a temperature sensor? If not, what is TS tied to?


    -Joe
  • 0 in reply to Joe Stephan
    Expert 1110 points
    Hi Joe,

    Thanks for your answer. I have been trying to confirm that this problem happens all the time. And yes it always happens. It is noticeable especially when the battery is almost empty. The charge current measured by a multi-meter in the path of the battery starts from 25mA, although I have set the CC current to 60mA. And as a result the charging process ends sooner than expected.

    I also disabled the VIN_DPM, which I believe is for Dynamic Power Path Management, by setting the 7th bit in the VIN_DPM_AND_TIMERS register. But still the same problem.

    My Input voltage is provided by USB. I have an FTDI UART to USB converter and in its EEPROM settings I have set the maximum allowable current to 300mA. So I believe the USB should be able to provide the current.

    Also my system shouldn't consume more than 10mA. Let's say 50mA (which never happens I believe). About the temperature sensor, no I don't have a temperature sensor and TS is connected to VIN.

    And do you know the answer to my questions?
    1- Should the profile be changed from CC to CV when the battery voltage is equal to regulated voltage?
    If so,
    2- Is the value in VOLTAGE_BASED_BATTERY_MONITOR register used for switching from CC to CV?
    3- Does the chip considers the battery voltage with charger connected? Or it has an internal way of disconnecting the charger and measuring the battery?

    Vala
  • 0 in reply to Vala
    Expert 1110 points

    As a numeric example, I just measured the battery voltage with charger disconnected when I noticed the Charge done flag has set in the status register. And it was 3.85V (the regulated voltage is 4.1V).

  • 0 in reply to Vala
    TI__Intellectual 2050 points
    HI Vala,

    To answer your questions:

    1- The profile should tranistion to CV from CC once the battery voltage has reached VBATREG setting.
    2- VOLTAGE_BASED_BATTERY_MONITOR is used to monitor the discharge of the battery, so this setting should not affect charging.
    3- The chip uses a system of internal comparators to monitor the voltage level of the battery, it does not disconnect the battery when reading.

    What is the battery you are using?

    -Joe
  • 0 in reply to Joe Stephan
    Expert 1110 points
    Thanks for your explanations Joe. The battery that I am using is a Li-ion 120mAh battery. Unfortunately no more information since they come from China. I will do more testings with other batteries are other devices. I will also move the current measurement setup to the USB side (VIN) instead of putting it in the path of battery.
  • 0 in reply to Vala
    TI__Intellectual 2050 points
    Vala,

    Are you using the TI bq25120 evaluation module? Or is this on your own board?

    Are you able to share a schematic showing just the bq25120 and the surrounding components?

    -Joe