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BQ77915: BQ7791502 Schematic review

CHO SR
Genius 3415 points
Part Number: BQ77915

Hi, 

The customer uses BQ7791502 to develop the BMS.

During the lithium ion battery 4-cell charge test, I was unable to charge it, so please review the circuit as well.

* BMS Setup Specifications

- Chemistry(Capacity) : Li-ion(1.9Kw)

- Cell balancing type : External FET

* Configuring Batter Pack

- 18650 2500Ah Li-ion : 4S45P

* Schematic Notes *

1. The NC treated part is not soldering.

2. Changed to CBO flow and CBI low.

3. Connects to VR1 = 750k, CCFG = AVDD.

  • 0
    TI__Guru* 81034 points

    Hi,

    Can you tell me the state of the CHG and DSG FETs - are they both open or only the CHG FETs? When you first connect the battery, if the battery is already fully charge it may prevent the CHG FET from turning on. The device assumes an OV fault after reset, and clears automatically after an OV delay if all cell voltages are below the OV threshold minus hysteresis.

    Regards,

    Matt

  • 0 in reply to Matt Sunna
    TI__Guru* 81034 points

    Hi, 

    A couple more observations on the schematic. The P-ch for turning on the charge FETs is backwards (FET6) and will prevent the charge FETs from turning on.

    Since you have 18 FETs in parallel with gate resistance up to 11.4 nF each, the turn off will be slow and turn on will likely be slow too. 

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Genius 3415 points

    Hi, Matt

    Thank you for your support.

    Customer review results: It has been identified as a GND pattern problem.

    The customer asked additional questions.

    1. If the charging voltage is above approximately 17 volts, the CHG and DHG signals from BQ7791502 will be lost. Is this symptom normal?
    - The battery is not fully charged.
    - It will then operate when the battery and charger are removed and reconnected (charging 17 volts or less)

    2. Battery charging current is less than 0.9A max. How do you increase the current?

    3. The UV will not shut off when discharged.
    - 1 cell went down to 2.6V at the UV 2.9V setting.

    4. Approximately 0V is measured on Vtb, pin 19 of BQ7791502.
    -Vtb does not output voltage and Vts does not detect voltage.

  • 0 in reply to CHO SR
    TI__Guru* 81034 points

    Hi,

    Have you made any additional progress on this debug. I apologize for the late response - I was out of office for the holidays and returned today.

    1. This is not normal (CHG and DSG turning off when voltage is above 17V). The OV condition for this device is a cell voltage above 4.2V and only the CHG FET should turn off. An OCC protection will turn off both the CHG and DSG FETs. The OCC threshold is 70mV across the sense resistor.

    2. This is limited by the OCC threshold. The sense resistor would need to be a large value to trigger this (70mV threshold) at such a low current.

    3. The DSG FET should turn off when the cell voltage falls below 2.9V. 

    4. You should be able to observe an occasional pulse on the TS pin when the temperature measurement is taking place.

    Best regards,

    Matt