I got a free laptop that has a stuffed battery so I thought I would tear it down.
The BQ9000 is a custom gas gauge ic with no public datasheet. It may have signature validation of the eeprom to stop 3rd parties making compatible batteries.
https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/415037/looking-for-data-on-communicating-with-bq9000
https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1071541/bq40z50-srec-programming-for-bq9000-bq9002-and-bq9004-generation
The shunt resistor is small, so the losses must be low meaning the resistance must also be low.
The thermal fuse over the MOSFETs
Back to back MOSFETs
