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BQ76940: bq76940+bq78350+bq76200,the charge current is too low

user5811580
Intellectual 700 points
Part Number: BQ76940
Other Parts Discussed in Thread: BQ78350, BQ76200, , BQSTUDIO

The BMS is made up of bq76940,bq78350 and bq76200.

Everything is ok.

Now, the charge voltage is 65V and there are 15 batteries at all.

However, the charge current is too low, and it's only almost 30mA.

What's the problem?

The registers of bq78350 are listed as below.

  • 0
    TI__Guru 54715 points
    Hi Wu,
    The current does seem low, if your 15 cells are about 59V as shown above and the applied charger voltage is 65V, for 27 mA the path resistance would be about (65-59)V/.027A = 222 ohm. Something seems off.
    If both FETs are off and you are charging the DSG FET would act as a diode dropping perhaps 1V. However 27 mA seems like a lot of leakage if the charge FET is off. Measure to find the voltage drop, that should indicate where to look for the issue.
    Check across various connections and components with a meter to find the voltage loss, perhaps something is not soldered or connected tightly.
    The display shows the CHG and DSG should be enabled. Check voltage of DSG and CHG out of the bq76940 to see that it is high. Verify they make it to the DSG_EN and CHG_EN of the bq76200. Also check the DSG and CHG of the bq76200 with respect to battery+, and across the gate-source of the charge and discharge FETs.
  • 0 in reply to WM5295
    Intellectual 700 points

    Everything seems OK.

    Just have a look at the voltages listed as below:

    Do you have any suggestions?

  • 0 in reply to WM5295
    Intellectual 700 points
    In fact, when a load is added without the charger, the load can work well.
    Besides, the load current can be up 1A during the testing process.
  • 0 in reply to WM5295
    Intellectual 700 points
    The LED should flash during the charging process as LED_EN has been enabled.
    However, the LEDs are all off during the process.
    It's very strange.
  • 0 in reply to user5811580
    TI__Guru 54715 points
    Hi Wu,
    From the voltages the pre-charge FET is on, the charge current will be limited by the parallel combination of the resistors (R607...), about 300 ohms. Looking at the bqStudio register view in this condition should show the PCHG bit in Operation Status set rather than CHG. Check the cell voltages and configuration of the precharge voltage thresholds in the bq78350 technical reference manual (TRM). The TRM was updated recently, please be sure you use the latest one. The update clarified a number of confusing points.
    The LED configuration is set with configuration bits, see section 13 of the TRM. In addition to the LED_EN be sure you have LEDCHG set.
  • 0 in reply to WM5295
    Intellectual 700 points
    Hi,I have checked the pre-charge settings,the pre-charge threshhold is 2500mV and the recovery threshhold is 2900mv. Now, all cell voltages are higher than 3800mV.Besides,LEDCHG has been set.It's very strange.
  • 0 in reply to user5811580
    TI__Guru 54715 points
    Hi Wu,
    Try sending a reset, some settings do not take effect until after reset.
  • 0 in reply to WM5295
    Intellectual 700 points
    I Have made A test with the system rebooted.However,I Just get the same result.It's very strange.
  • 0 in reply to WM5295
    Intellectual 700 points
    I have checked all settings. The charging current is made up of pre-charge and charge . Just remove Q603 and the charging current will drop to 10mA. I just can not understand why the system enters pre-charge state. Besides, I have gauged all cell voltages and they are all around 4.0V.
    Do you have any other suggestions?
  • 0 in reply to user5811580
    Intellectual 700 points
    Can anyone give any suggestions?
    The settings in bqstudio seem ok.
    But the charge current is to low, only around 40mA.
    The cell(1..15)voltage is almost 59.4V and the charge voltage is almost 65V.
  • 0 in reply to user5811580
    TI__Guru 54715 points
    Hi Wu,
    Be sure the bqStudio display gets updated.
    Check all the reported cell voltages to be sure they are not in the precharge range.
    Check for broken components or solder joints.
    If the display shows something different than what you measure, something must be mis-configured or broken.
    You might re-load the firmware, then make configuration changes from default slowly to see where the problem occurs.
  • 0 in reply to WM5295
    Intellectual 700 points
    Hi,in fact,I have enabled "scan" in bqstudio.
    I have gauged cells' voltage manually For many times.I'm sure all cells' voltage are over 3.8V,which is higher than the precharge's uppon limit. Just reload the default firmware downloaded from ti.com? Is it necessary to discharge the batteries' voltage downto 3.2V？In fact,the batteries' voltage have not been lower than 3.8V.Besides, the system Can work well in the discharge process and the discharge current Can Be over 2A.
  • 0 in reply to WM5295
    Intellectual 700 points
    During the charge process,the current from the "fast" charge seems constant,which is almost 10mA.When reducing the resistors' combination(1.2K), the charge current will arise.However, bqstudio just show "FCHG".Besides,the LEDs are not on during the process,which can be on during the "boot" process.LED_EN and LEDCHG have been set. Does this means the process is pre-charge?
  • 0 in reply to WM5295
    Intellectual 700 points
    Thank you very much.
    In fact, the N-MOS has been destroyed.
    Just change a new N-MOS and everything is OK.