We have made our Battery Management System using a BQ40Z80 chip with External cell balancing according to the schematic from TI, the one shown below
On 5 of our BMS, when reading the values on the BQ40Z80 chip from the Battery Management Studio, the cell 1 voltage shows an incorrect value of around 80mV but all other cells show 4150mV. When measuring with a multimeter the voltage on cell 1 reads 4.15V so we ruled out that it was a faulty battery.
Also, we measured the voltage at VC1 and the reading was 4.15V.
The thing to note is that these BMS worked fine at first, but suddenly, and with no apparent reason to us, the BQ chip started to read an incorrect value.
The latest one that stopped working was used for making a golden file. Logging the charge, relax, discharge, relax process for obtaining the ChemID was finished without any issues. The next day, when we wanted to start the learning process, we first charged the battery without the BMS (as the documentation shows that the first step of the learning process was a discharge). When we connected the balancer cable of the battery to the BMS the BQ chip wouldn't wake up as it went into shutdown mode when it read a cell voltage lower than Shutdown Voltage. We woke up the BQ chip manually by connecting batt+ and out+ until we lowered the Shutdown Voltage value so that the BQ chip wouldn't go into shutdown mode.
Is this a known issue? Could it be a hardware or firmware problem. All cell balancing options from the data flash were turned off on this BQ40Z80 chip and the calibration for Current, Applied Cell 1 Voltage, and Applied VC6-VSS Voltage were done before the logging for the ChemID.
Also, some BMS boards have not had this problem at all and still after one month continue to read correct cell voltage values.
Thanks in advance.