data/text/DLP2/891022

BQ20Z655-R1: Voltage Error

Part Number: BQ20Z655-R1 Customer is bringing up 4S1P battery pack design using BQ20Z655-R1. They are seeing the voltage reading on each of the 4 cells droop in software (BQ Evaluation Software). Each cell is at 3.43V (all within 5mV). When they do a software reset (Manuf Access Code 0x41), then SW will read the appropriate voltage, but quickly droop to something like 2V on an exponential decay. Even when overriding FETs to allow 1A charge, they still see the individual cell voltages drop after a reset. They confirmed cell measurements (and at input to IC) are all consistent with 3.43V (under rest) and >3.5V charging. Customer is using BQ20Z655-R1 on 3S1P pack and it works fine. To note, they have not written the GG file yet, but am working on that now. Any guidance you can provide would be greatly appreciated! Regards, Mark

Hi Mark, Did the customer use an EVM? Did they do the calibration? Andy

Hi Andy, Apparently EVMs are no longer available. That was our first thought. What’s the easiest method for doing the calibration? Regards, Mark

Hi Mark, See Section 9 of the bq20z655EVM User's Guide. Andy

Thanks Andy!

Hi Andy, The customer tried calibrating albeit with cells attached and monitoring the overall voltage (each cell is pretty tightly balanced). And immediately after calibration, voltages seemed okay, but again drifted apart. I’m curious how calibration might resolve the voltage decay issue. What else should we try? BQ20Z655EVM is EOL. Please advise. Thanks, Mark

Hello Mark, Can the customer send a log file of the voltage drift after calibration?

Hi Kang, Attached is the log from immediately after calibrating. Regards, Mark voltage testing.log

Hi Mark, I would suggest your customer try a resistor cell simulator instead of battery cells and see if the same issue still occurs. See the screenshot below. Andy

Hi Andy, Sorry for the delay in responding. The customer tested using 10K resistors, but guessing that shouldn’t matter. What else can they try? Do you notice anything unusual in the logs? Regards, Mark

I also checked the log file. Besides the voltages that drop, there isn't anything in the log that sticks out. But yes, these voltages are troublesome. Is that just on one board or is that a problem with several boards/gauges?