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BQ34Z100 PACK- connection to sense current

Chang Fatt Lee
Prodigy 30 points
Other Parts Discussed in Thread: BQ34Z100, BQ24650

Hi TI,

I am quite confused with the Ground connection of bq34z100 fuel gauge. I am using Polymer Lithium battery pack with only 2 terminals. May I know how do I sense the current with PACK- terminology?

1. I have checked from the bq34z100 typical implementation. How do I connect to PACK- if my battery does not have this terminal?

2. After this, my battery will be charged by the Solar charging IC – bq24650. For this configuration, will the PGND connect to Ground? But until here, there is no PACK- involved too. Please advice.

3. Can you please show me the new drawing based on the reference design of bq34z100? (battery with only 2 terminals)

Thanks.

 

Lee

  • 0
    TI__Expert 6470 points

    Hi Lee,

    The gauge requires that you have the sense resistor in place to measure the current. If you are using a commercial pack, then the sense resistor should be placed close to where you are putting the bq34z100 on the host side of things.

    You will want to arrange the charger connection so that its current also passes through the same sense resistor.

    Regards,

    Doug

  • 0 in reply to Doug Williams4811
    Prodigy 55 points

    Hello, I am having the same doubt, my battery only has BAT+ and BAT- (1 Cell Li Ion), and I don't really know how to connect the sense resistor. I had supposed it could be in series with the battery current, but tied in between the BAT- terminal and its ground. Regarding what you said, would it be sensible to place it in between the SRN and SRP terminals (very close to the IC) in series with BAT- terminal? Would I obtain correct readings?  I would connect the charger, too.... But I still have to figure out the design.

    Thanks!

  • 0 in reply to Ariel Alonso
    TI__Guru 73830 points
    The sense resistor must be connected between BAT- and the negative potential of your load or charger. Charge and discharge current must flow through the resistor and the SRN and SRP pins must be connect across it through the RC filter.
  • 0 in reply to ThomasCosby
    Prodigy 55 points

    Thank you Thomas, that's how I am trying to connect everything. I have a Battery Charger Module (the "charger", green section), a 3.3V Regulator Module (the "load", red section) and the Fuel Gauge circuit (yellow section). This is how I connected everything. Is the sense resistor connected properly? I have some doubts if this will alter the functionality of other modules or the I2C communication (I am missing the pull up resistors there yet I believe). My 1cell Li Ion battery has 3 pins, BAT+, BAT- and Thermistor. Thanks in advance!

  • 0 in reply to Ariel Alonso
    TI__Guru 73830 points
    We normally connect the bq34z100 and its grounds to the BAT- side of the sense resistor at negative terminal of the cell. That way the load of the device does not affect current measurements. I would also run the SRN signal path as a Kelvin contact to the sense resistor, instead of the the GND plane. The GND plane connection will affect curent measurements.