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BQ27750EVM-837: BQ27750EVM-837 interacts with Electronic load: how to open FET

Wenlong Wang
Intellectual 275 points
Part Number: BQ27750EVM-837
Other Parts Discussed in Thread: BQSTUDIO, BQ27Z561EVM-011

Hello,

I am trying to make BQ27750EVM-837 work together with an electronic load (RIGOL DL3031).

After ensuring the electronic is full functional, I connect one BQ27750EVM-837 module to the electronic load. However, when I try to discharge my battery and get measurement through battery gauge, the voltage between Charger+/Load+ and Charger-/Load- is much smaller than the battery voltage. In addition, the current is much smaller than expected as shown in the photo below:

There is some information about my system:

1. Battery gauge is connected to PC and PC is able to recognize the EVM.

2. I use bqStudio to operate the EVM. I open the FET and make sure "FET_EN" bit of bq27750 is set high.

3. The target discharge current is 1A. I have already checked on the same battery without the battery gauge and ensure all the settings on electronic load are correct.

4. I have already pressed S1 button on the EVM before using it and guarantee the battery gauge IC is turned on.

5. My battery is NCR18650B. Before this experiment, it is fully charged. The voltage of the battery after charging is 4130 mV.

In my analysis, the problem is caused by that FET is not able to be turned on completely. Could you give me any suggestion about my current situation?

Kindly regards,

Wenlong  

  • 0
    TI__Guru* 76230 points

    Hi Wenlong,

    What voltages does the chg and dsg pin? Can you take a screen shot of bqstudio and attach.

    thanks
    Onyx

  • 0 in reply to Onyx Ahiakwo
    Intellectual 275 points

    Hi Onyx,

    the voltage between chg and dsg pin is 315.7mV.

    Here is the screen shot of bqstudio. If there is any information missing, please let me know.

    Thanks.

    Wenlong

  • 0 in reply to Onyx Ahiakwo
    Intellectual 275 points

    Hello Onyx,

    do you have any idea about this problem?

    Kindly regards,

    Wenlong

  • 0 in reply to Wenlong Wang
    TI__Guru* 76230 points
    Wenlong
    Did you modify the bq27750 EVM? The gate of the charge and discharge fets do not have the needed voltage to turn on. Can you program the gauge with the default srec so we ensure that it is not something that you configured in the DF that is causing this. If that doesn't cause the chip to correct, you may need to replace the gauge IC? Perhaps that is bad chip
    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Intellectual 275 points
    Hello Onyx,

    I reprogram the gauge using default srec file. I try both bq27750_v0_04_build_12.srec and bq27750_v0_05_build_13.srec with steps described below:
    1. After firmware programming completes successfully, I close bqstudio and open it again.
    2. Both the gauge IC and programmed firmware are distinguished by bqstudio. I select Battery Chemistry (0x2012) and leave all the other settings as default.
    3. Then I send RESET command to gauge.
    4. After all the register information displayed correctly, I send FET_EN command from command panel.
    5. Then I see FET_EN, CHG and DSG bits are set to 1. At this time, I tried to discharge one battery cell via 1A constant current by a electronic load. But the information from electronic load front panel shows that the discharge process fails.

    Before all the procedures above, I test the battery and electronic load. It is ensured that both of them works.

    Could you give me some advise on what shall I do in order to make the EVM work? I bought it one month ago. I follow all the steps from the user manual strictly. However, till now this EVM doesn't work even once. In recent days, my lab would like to buy another two bq27z561EVM-011 from TI store. It is really important to at least to make this one work before purchasing the other two.

    Considering the package of the battery IC, it is not feasible in my case to remove the gauge. Actually, the reason why purchasing EVM instead of IC is that we don't want to solder battery gauge circuit by hand. If there is any other information you need about the test I do on the gauge EVM, please let me know.

    Thanks,

    Wenlong
  • 0 in reply to Wenlong Wang
    TI__Guru* 76230 points

    Hi Wenlong,

    The fact that the charge and dicharge fet gates are not seeing the needed voltage is indicative of a bad IC.

    I would have liked to see a scope shot of Vbat, Pack p, and the chg and dsg gates so we can verify and see what is happening with the fets

    thanks

    Onyx

  • 0 in reply to Onyx Ahiakwo
    Intellectual 275 points

    Hi Onyx,
    just to ensure the content of scope shots I should take:
    1. Vbat and GND
    2. Pack and GND
    3. Chg and Dsg
    These scope shots should be taken when the gauge IC is turned on.
    Anything else needed?

    Thanks,
    Wenlong

  • 0 in reply to Onyx Ahiakwo
    Intellectual 275 points

    Hi Onyx,

    this is the scope shots you required:

    1. Vbat

    2. Pack p

    3. Chg and dsg

    Kindly regards,

    Wenlong

  • 0 in reply to Onyx Ahiakwo
    Intellectual 275 points
    Hi Onyx,

    any advise from the scope shots I sent?

    Kindly regards,
    Wenlong
  • 0 in reply to Wenlong Wang
    TI__Guru* 76230 points

    You were to measure chg to gnd and dsg to ground.

    Why is the gate signals pulsing? I believe you have a defective IC. Either you replace the IC or purchase a new EVM.

    thanks

    Onyx