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bq34z100 has unreliable SoC when there is high current on start up

Russell Ellis
Prodigy 20 points
Other Parts Discussed in Thread: BQ34Z100

We are using a bq34z100 to gas gauge a 3 cell Li ion battery. Our circuit is very similar to Figure 8 Multi-Cell and 5-LED Display in the bq34z100 datasheet. The usual current draw for our product is about 750mA and the max current draw is about 2A.

We have noticed some inconsistencies with the remaining capacity measurement when we first connect a battery that get worse as the current draw increases. When I power my product using a DC supply so there is no current draw on the battery, I get the same remaining capacity reading every time I plug in a battery. If I have a normal load of 750mA drawn on the battery, the remaining capacity measurement will be within a 10% spread when I connect a battery. When I draw 2A the reading is quite unreliable.


I am pretty sure this has to do with the IR drop across the battery when the OC voltage reading is taken by the fuel gauge. My question is, is there a mode that I can set in the bq34z100 that will make it work even when current is being drawn from the battery at start up.?

The other option I am exploring is to start the fuel gauge before my power supplies, but I would like to avoid hardware changes if possible.

-Russ

  • 0
    TI__Guru 73830 points

    Russ, 

    The device runs an FCC simulation when a reset occurs due to a cell connection event or a Reset (41) command. The device will take an OCV measurement to estimate the capacity of the cells. If the cells are under load, then the OCV measurement will not be valid and the capacity prediction will be low. It is not a valid condition to reset the device under load and get an accurate capacity estimate. This is not a problem on our gauges that control the FETs, because they will not turn the FETs on until the device initialization has completed. The bq34z100 does not control the FETs, so the FET controller should keep the FETs off until the system has been initialized. The capacity prediction should get corrected as the discharge continues and the device runs another FCC simulation at an Ra grid point. 

    Tom

  • 0 in reply to ThomasCosby
    Prodigy 20 points

    Thanks for your quick response. I thought that this would be the case. Just to make sure that I understand you, I should try to have as low of current as possible on the battery when the fuel gauge is reset. How long after a reset should I wait before turning on the FET controller?

    One other question about the capacity prediction getting better as the battery discharges. From what I've read, the fuel gauge will not take a new OCV reading until the discharge current is less than the quit current for 6 minutes, and the quit current needs to be less than C/20. There are many cases where the battery will be discharged at a rate greater than C/20 for the entire discharge cycle. Will the fuel gauge still improve its capacity prediction as the battery discharges?

  • 0 in reply to Russell Ellis
    TI__Guru 73830 points

    It usually takes 2 to 3 seconds for the gauge to initialize and run the simulation.

    The gauge does not take OCV readings during discharge, but it does read the battery voltage every second. It uses this measurement, current, Ra table and temperature to simulate a discharge curve to compare it the OCV curve and predict the capacity. It will adjust FCC and RM at the Ra grid points to fine tune the capacity to the Term Voltage.