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Original question:

BQ35100EVM-795: Battery Management Studio not working with BQ35100EVM

BQ35100EVM-795: How to measure uAmps with BQ35100EVM and BQStudio

Vladimir Zitoli
Prodigy 20 points
Part Number: BQ35100EVM-795
Other Parts Discussed in Thread: BQSTUDIO, BQ35100

Dear all,

I am using BQ35100EVM-795 EvalBoard in order to characterize the current absorptions of a PCB board supplied by a Li-SOCl2 battery by using BQSTUDIO.

The current range is 20uA - 40mA, so I decided to solder a 3.3 Ohm resistance as sense resistor on the EvalBoard.

As a matter of fact, in this way I could measure from 2.3uA ( = 0.125 V/ 3.3R /(2^14-1) ) up to 37.8mA ( = 0.125V/3.3R).

After having calibrated the fuel gauge, I decided to test his accuracy before use it on my board.

However, I noted that:

1. The minimum measured current is 1mA. With currents lower than 1mA, it measures 0mA

2. The current offset error is too high (it counts 2uAh every minute at no load condition, that is more or less 100uA of offset current), even if the calibration procedure was OK. How can I minimize this error?

Looking forward to hearing from you soon.

Kind regards,

Vladimir Zitoli

  • 0
    TI__Guru** 102555 points

    Hello Vladimir,

    You will need to scale the current so that each 1mA represent a smaller value, for example perform the calibration with 10mA flowing on the 3.3 ohm resistor but tell the gauge it is actually 1A (this example everything would scale by 100) so when you would read 1mA it would actually indicate that 10uA is flowing.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 20 points

    Hi Wyatt,

    thank you for your reply.

    I calibrated the EvalBoard as you suggested but now I see an offset error (21mA more or less), even if I calibrated that through the "CC Offset" and "Board Offset" calibration.

    In particoular, I see that right after the offset calibration, it reads an offset current of 0mA, but later it return to measure -21mA.

    In attached, the screenshots of the "Watch" section of BQSTUDIO before and after the offset calibration.

    How can I delete the offset error? The scale factor I used during calibration is 100 (32mA <--> 3200mA)

    Looking forward to hearing from you soon.

    Kind regards,

    Vladimir Zitoli

  • 0 in reply to Vladimir Zitoli
    TI__Guru** 102555 points

    Hello Vladimir,

    You may need to increase the deadband register since a very small current actually corresponds to a much larger read from the gauge. 1LSB is large amount with increased gain.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 20 points

    Hi Wyatt,

    Thank you for your reply.

    I didn't find this register neither into bqstudio nor into the bq35100 datasheet and User Guide. Could you show me where can I find it please?

    Thank you in advance.

    Kind regards,

    Vladimir Zitoli

  • 0 in reply to Vladimir Zitoli
    TI__Guru** 102555 points

    Hello Vladimir,

    I took a closer look and the BQ35100 does not have the same type of deadband functionality as our other gauges, do you always measure the offset in the same direction? (negative 21mA)?

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 20 points

    Hi Wyatt,

    yes I see a constant offset current of -21mA.

    Kind regards,

    Vladimir Zitoli

  • 0 in reply to Vladimir Zitoli
    TI__Guru** 102555 points

    Hello Vladimir,

    I would make sure the load you use if very precise. at least 1mA in order to make sure the gain is as close as possible. You will need to use the offset calibrations to offset the 21mA.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 20 points

    Hi Wyatt,

    I used an active load during calibration process.

    Anyway, the issue comes up during the CC Offset calibration, as the eval reads ad offset current of -21mA event though there is no current flowing into the sense resistor.

    What can I do in order to minimize the offset current error?

    Looking forward to hearing from you soon.

    Kind regards,

    Vladimir Zitoli

  • 0 in reply to Vladimir Zitoli
    TI__Guru** 102555 points

    Hello Vladimir,

    Can you share your calibration information? Also these tests are using the EVM with 3.3Ohm resistor with low PPM for testing?

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 20 points

    Hi Wyatt,

    I use this instrumentation during the calibration:
    1. Power supply: 3.6V 80mA

    2. Active Load in order to guarantee a constant current of 30mA or 40mA during current calibration

    The sense resistor is a SMD 1206 3.3R resistor with a 1% resistance tolerance. This kind of tolerance is suitable for my application (to me, is important to minimize the offset current as my application is in sleep mode most of time with few uA of current absorption).

    I had followed the calibration procedure showed in this TI document: SLUUBH7 (bq35100EVM-795 Evaluation Module User’s Guide) 

    Let me know if you need more detailed informations.

    Kind regards,

    Vladimir Zitoli

  • 0 in reply to Vladimir Zitoli
    TI__Mastermind 23091 points

    Hi Vladimir,

    Wyatt is out of office. He will be back on Monday.

    Regards,
    Jose Couso

  • 0 in reply to Jose Couso
    TI__Guru** 102555 points

    Hello Vladimir,

    If you are using this in ACC mode, I would not recommend trying to capture all passed charge, scaling the sense resistor can be used to get higher resolution  on small loads but the current consumption of the gauge will be very high if the ACC is always running. ACC mode is generally intended for use of capturing small higher current pulses then using assumed standby current when the system is not active.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 20 points

    Hi Wyatt,

    thank you for the reply.

    So if I understood well, you are saying that BQ35100 reads the itself current absorption in ACC mode. Is it right?

    Looking forward to hearing from you soon.

    Kind regards,

    Vladimir Zitoli

  • 0 in reply to Vladimir Zitoli
    TI__Guru** 102555 points

    Hello Vladimir,

    In ACC mode the current consumption is high because we are in a fully active state and during the charge accumulation.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 20 points

    Hi Wyatt,

    The question is the following one:

    Is the offset current read on the GUI related to the BQ35100 absorption in ACC mode? If yes, how is it possible since the sense resistor is not connected to the bq35100 supply pin?

    Looking forward to hearing from you soon.

    Kind regards,

    Vladimir Zitoli

  • 0 in reply to Vladimir Zitoli
    TI__Guru** 102555 points

    Hello Vladimir,

    The current consumption of the gauge should not be over 1mA even when fully active, but with your scaling factor it may be reading the gauges own power consumption.

    Sincerely,

    Wyatt Keller