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Original question:

BQ77915: BQ77915: 8-series cell configuration with BQ77915

BQ77915: PACK- vs Local GND Connections

Miguel Hennemann
Prodigy 30 points
Part Number: BQ77915
Other Parts Discussed in Thread: BQ24610, BQ34110

Hello,

1. Image 1 is from the BQ77915 datasheet. Are the blue circled "ground symbols" actually local grounds that are basically BAT- ? 

2. Is PACK- just the overall ground? Please look over image 2 and verify my connections. I have circled overall ground as red in image 2.

3. We are using the BQ24610 as a charger that will connect to the BQ77915 circuit. In image 3 I have circled in yellow some the the overall grounds in our BQ24610 schematic. Are these overall grounds also connected to PACK- ?

Thanks,

MH

  • 0
    TI__Guru* 80843 points
    Hi Miguel,

    You are correct, the GND symbols in the datasheet schematic refer to BAT-. In battery packs, there is often a need to have separate battery and system grounds. Here is a good thread on that topic: e2e.ti.com/.../585790 The EVM User Guide has a more detailed schematic that might be more helpful: www.ti.com/.../sluubu2

    I do not have experience with the charger IC, but I believe this should reference PACK-.

    Best regards,
    Matt
  • 0 in reply to Matt Sunna
    Prodigy 30 points

    One more question pertaining to my grounding concern.

    We are trying to measure the voltage of the battery pack and we are using a simple voltage divider to scale the output from the batteries to something under 5V, which is suitable for the micro-controller we are also powering from the battery pack. 

    Would using the input of the battery pack as a power source and as a voltage sensor output be a concern, especially considering we are connecting those grounds to the "System Ground". Would this cause any ground loops? The only other option would be to connect to BAT-. So between "System Ground" and BAT-, "System Ground" seemed like the more reasonable choice. 

  • 0 in reply to Miguel Hennemann
    TI__Guru* 80843 points
    Using a resistor divider is going to pull a constant current from the battery which may run down your pack over time.
    I'm not sure if this would cause a ground loop - it does seem like it could create a undesirable current path. A good video that might give some helpful ideas is located here: training.ti.com/bq34110-and-bq34210-q1-battery-gas-gauges-rarely-discharged-applications
    The BQ34110 is a gauging IC that monitors the battery voltage, so the schematic in the video addresses some similar challenges to those you are facing.