To cover a 16s battery cell system or greater, two BQ769x2 devices can be cascaded to monitor up to 32s battery cells. This design tests two BQ76972 devices to monitor up to 32s battery cells. The bottom BQ76972 monitors the lower 16s battery cells and the top BQ76972 monitors the upper 16s battery cells, so the bottom BQ76972 shares the same ground with BAT– and MCU while the top BQ76972 references 16s stack voltage. Adding isolation is required when communicating with the top BQ76972 device or a discrete level shifter can be used here. This design uses an I2C isolator, ISO164x, for up to 400kHz I2C communication baud rate and low power consumption. Using the discrete level shifter is acceptable for other signals like ALERT, RST_SHUT, DFETOFF, CFETOFF, and so forth, since these signals are not acting frequently. MCU issues commands and reads voltage, current, and temperature data from the bottom BQ76972 directly and through ISO164x when communicating with the top BQ76972.

For the faults in the upper 16s battery cells, the top BQ76972 detects the faults and drives MOSFET off directly. The MCU can be made aware through ALERT or reading status registers and turn on Q65, to make sure DSG MOSFET is completely off. For the faults in the lower 16s battery cells and current faults, the bottom BQ76972 detects them and informs the top BQ76972 to drive MOSFET off. For slow protections, like COV, CUV, OT, UT, OCD1, OCD2, alerting the MCU is acceptable when faults are triggered and the MCU then issues a command to turn off MOSFETs. While for short-circuit protections, which normally requires µs delay time, the process is not fast enough if leveraging MCU firmware for the protections. This design adds discrete circuits to allow the bottom BQ76972 device to control MOSFET directly with the top BQ76972 device, avoiding further protection delay caused by MCU firmware.