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BQ25750: Battery discharge current is reading zero

Teun Noordkamp
Prodigy 50 points
Part Number: BQ25750


Tool/software:

Hello there,

At the moment we are testing our hardware. During these tests we found out that the battery current meassured a value of 0mA during discharge of the battery, during charging everything seems fine. All the other ADC channels are reading out fine during discharge. So what are we doing wrong? We almost use the same circuits as shown in the datasheet (our load is connected to the high side of the battery shunt resistor and we removed BATFET [Q3,Q4] as they were optional), we expected to measure the current discharging the battery this way, as the internals switch the internal power automaticly between VAC and SRN. The ADC is enabled and running in continous mode, the IBAT channel is enabled just like all the other channels. Is there something we are missing? Could you please help us to figure out why this function isn't working?

Kind regards,

Teun

  • 0
    TI__Genius 9120 points

    Hi Teun,

    Are you discharging in reverse mode or directly through the BATFETs?

    Best regards,
    Michael

  • 0 in reply to Michael Bradbourne
    Prodigy 50 points

    Hi Michael,

    Thanks for your quick reply. No, I'm not using the batfats or reverse power mode. The load is connected to the top of Rbat_sns. I did this in response to a previous question I posted on the TI forum

    (https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1486113/bq25750-as-a-solar-battery-charger/5709323?tisearch=e2e-sitesearch&keymatch=%25252520user%2525253A628774#)

    I hope this makes sense.

    kind regards,

    Teun

  • 0 in reply to Teun Noordkamp
    TI__Genius 9120 points

    Hi Teun,

    Thanks for the clarification. I think the issue is that the ADC cannot measure reverse current unless the device is in reverse mode.

    Best regards,
    Michael

  • 0 in reply to Michael Bradbourne
    Prodigy 50 points

    Allright, but I am kind of puzzled why would this would be a feature instead of a continuous ADC measurement that work despite the other functions of this chip. This also means that if the controller isn’t in reverse power mode and thuss using the batfets it couldn’t read the current.

  • +1 in reply to Teun Noordkamp
    TI__Genius 9120 points

    Hi Teun,

    The current sense amplifier is unipolar. This means that something has to flip the polarity for it to work. In this case it would be the reverse mode bit. Reading only 0 mA current would be expected because we would be clamped at the zero value for our lowest reading. I know that the reverse mode will flip the polarity for the current sense amplifier. I don't normally work with the power path device in this family of devices, so I can ask the primary expert if turning the BATFETs on would allow for discharge current too when they return to office.

    Best regards,
    Michael

  • 0 in reply to Michael Bradbourne
    Prodigy 50 points

    Hi Michael,

    Today we did some measurements. Our circuit is as follows:

    At V_SOLAR we connected a Lab bench power supply set on 10V 1.25A. At BAT+ and BAT- we connected a LiFePo4 battery who's voltage was 13.3V. The load it self drew about 170mA. Due to the regulation of the BQ25750 the voltage of the lab bench power supply dropped to 4.3V with the maximum current of 1.25A. 

    P_in = 4.3 * 1.25 = 5.375W

    I_out = 5.375 / 13.3 = 0.404A

    I_charge = 0.404 - 0.17 = 0.234A

    We then configured the BQ25750 so the watchdog was disabled (for debugging reasons) and the ADC was in single shot mode and the highest resolution. This gave us the following results:

    • I_AC = 0x03af > 943 * 2mA = 1886mA
    • I_BAT = 0x0054 > 84 * 2mA = 168mA
    • V_AC = 0x0824 >  2084 * 2mV = 4168mV
    • V_BAT= 0x19a9 > 6569 * 2mV = 13138mV
    • V_SYS = 0x0828 > 2088 * 2mV = 4176mV
    • Ts = 0x01de> 478 * 2mV = 956mV
    • Vfb = 0x05ce >  1486 * 2mV = 2972mV

    I also measured the voltage with a multimeter and I got the following values:

    • V_R3 = 2.5mV / 0.002 = 1.25A (as expected)
    • V_R4 = 1.2mV / 0.005 = 0.24A (as expected based on calculations)
    • V_AC = 4260mV
    • V_BAT = 13400mV
    • V_SYS = 4232mV
    • Ts = 1931mV
    • Vfb = 1489mV

    As you can see the ADC is working fine except for the currents and Ts (I exptected some diffrence like +/-5mA or +/-10mV). We expected it to be around 0.234A as calculated.

    We also tested to measure the reverse current with the reverse power mode enabled as you suggested. So we removed the lab bench power supply and set the reverse voltage at 5V (this is the POR value) and enabled it.
    We got a reverse current this time but it wasn't what we expected, as you know we would expect a value of about 170mA. We got 0xFFe5 which is -27 * 2mA = -54mA.

    So I know it's a lot of information and we got a reverse voltage so thank you, but why are the current values so far off? I again hope that you could help us.

    Kind regards,
    Teun

  • 0 in reply to Michael Bradbourne
    Prodigy 50 points

    That is realy good to know. I thought it was bipolar and would switch on it's own.

  • +1 in reply to Teun Noordkamp
    TI__Genius 9120 points

    Hi Teun,

    The ADCs are a good general indicator of the device parameters, but if higher accuracy is required, we recommend using external measurements from a battery monitor or another device where measurement accuracy is the main function. The control loops for the BQ25750 are separate from the measurement loops, and those are what the device prioritizes.

    Best regards,
    Michael

  • 0 in reply to Michael Bradbourne
    Prodigy 50 points

    Hi Michael,

    Thanks for your quick response during our problems, this helped us a lot and saved a lot of time.

    Kind regards,

    Teun