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 BQ76942 devices to monitor up to 20s battery cells for 60-V and 72-V battery packs. The bottom BQ76942 monitors the lower 10s battery cells and the top BQ76942 monitors the upper 10s battery cells, so the bottom BQ76942 shares the same ground with BAT– and MCU while the top BQ76942 references 10s stack voltage. Adding isolation is required when communicating with the top BQ76942 device or a discrete level shifter can be used here. This design uses an I2C isolator, ISO164x, for up to 400-kHz I2C communication baud rate and low power consumption. While 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, temperature date from the bottom BQ76942 directly and through ISO164x when communicating with the top BQ76942.

For the faults in the upper 10s battery cells, the top BQ76942 detects the faults and drives MOSFET off directly. The MCU can be made aware through ALERT or reading status registers and turn on Q47, to make sure DSG MOSFET is completely off. For the faults in lower 10s battery cells and current faults, the bottom BQ76942 detects them and informs the top BQ76942 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, it is not fast enough if leveraging MCU firmware for the protections. This design adds discrete circuits to allow the bottom BQ76942 device to control MOSFET directly with the top BQ76942 device, avoiding further protection delay caused by MCU firmware.