Hi,
I have board designed that uses a bq27520 as the fuel gauge. I have run through chemistry ID, learning cycles, calibration, and golden firmware generation.
I am running into issues when trying to measure the current being used on the device. I first program a firmware onto the main MCU on this board to set it up to shutdown all peripherals, enable the bq27520, and go into it low power state. I then connect to the bq27520 using the ev2300 programmer and TI battery management studio. I program the fuel gauge with my "golden binary," reset, enable impedance tracking, battery insert. I can then see my battery voltage and an approximate (%) of capacity and the measured current. At this point I want to run a calibration procedure on the new device. I follow the steps running a CC offset, Board Offset with the sense resistor manually shorted, and then I run the current calibration using a load tied to the 3v rail of my system. I pull 150mA from the 3v rail and measure the current being drawn from the battery. I run calibrate current.
I have had successful calibration of 3 devices using this procedure. I have failed with 11 devices.
What I mean by failing is that when I am connected to the device after flashing the "golden binary" and before individual device calibration. I can see the resting current as measured by fuel gauge to be approximately +30 mA. When measuring the current myself, the draw is approximately -1mA. When I apply the load, the current may drop a few mA's, but the device never detects a discharge.
With the devices that work, after flashing the golden binary, they will immediately properly detect ~0mA. When a load is applied, they will update properly see the negative current draw. These devices are able to be accurately calibrated.
My question is why am I seeing so many devices have this current measurement issue? I tend to think there is something wrong with the hardware setup (bc a few of them work), but I have not yet been able to identify any issues. The schematic is close to the same as the reference design, with a 50 mOHM sense resistor instead of a 10 mOHM like in the ref design. Even with a maximum current of 200mA, I don't think we'd reach the maximum input differential on that input: 50mOHM*200mA = 10mV. I believe the input maximum is +-125mV.
Thanks for the help,
Eddie
