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BQ21088: Battery management & fuel gauge (BQ27427) schematic review

Austin Friesenhahn
Prodigy 20 points
Part Number: BQ21088
Other Parts Discussed in Thread: BQ27427

Tool/software:

Hello, 
I would just like to verify that the following schematic connections will suffice for battery management and fuel-gauging of a lithium-ion battery pouch. I've added the rocker switch to turn the device off regardless of whether the battery or wall-adapter is plugged in.

Thanks for any help.

  • 0
    TI__Expert 8225 points

    Hello Austin,

    Thanks for reaching out. Overall, your schematic looks good. Please see my comments:

    • It appears that the DPDT rocker switch connects VBAT to the gauge and VBUS to the charger when switched on. I don't expect this will cause any issues for the charger. I'd also like to note that the BQ21088 includes Ship Mode and Shutdown mode, which can be entered via a push button press or I2C. These allow the device to enter ultra-low quiescent current states and are automatically exited when an input adapter is connected.
    • IN, SYS, and BAT each have the required capacitances.
    • I recommend using the external thermistor monitoring feature of the fuel gauge rather than the charger. The TS feature of the BQ21088 is tuned for a 103AT-2 thermistor (β=3435 and a characteristic resistance of 10kΩ at 25°C). If a different thermistor is being used, which appears to be the case, external compensating resistors need to be used on the TS pin. The fuel gauge allows for thermistor coefficients to be modified in RAM. I highly recommend using the fuel gauge's external thermistor monitoring as it's highly accurate. For reference, here's an app note on configuring the TS pin of the BQ2518x: https://www.ti.com/lit/an/sluaaj8/sluaaj8.pdf?ts=1739227471219&ref_url=https%253A%252F%252Fwww.google.com%252F
    • If you switch to using the fuel gauge for external thermistor monitoring, the TS feature of the BQ21088 can be disabled by tying a 10k from the TS/MR pin to GND. Then, the fuel gauge's temperature readings can be monitored to determine whether to enable charging via I2C.

    Let me know if you have any questions. I'll loop in the fuel gauge experts to review the BQ27427 portion of the schematic.

    Best regards,

    Alec

  • 0 in reply to Alec Lehman
    Prodigy 20 points

    Alec,

    Thanks for pointing out the thermistor type. We've switched to using 103AT-4-70261 instead and intend to use the fuel gauge for more accurate readings. I've also added a switch to TS/MR pin. I don't expect we'll need it given I2C capabilities but it's good to have for a prototype now. The prior NTC thermistor connected to TS/MR has been replaced with a 10k resistor. 

    Everything look good with these changes?

  • +1 in reply to Austin Friesenhahn
    TI__Expert 8225 points

    Hello Austin,

    Those changes look good. Feel free to reach out if you have any other questions.

    Best regards,

    Alec