Battery management system (BMS)

At minimum, a protection circuit disconnects the pack on over-voltage, under-voltage, over-current and short circuit. A full BMS also measures each series cell group, balances cells so they stay matched, monitors temperatures and blocks charging outside safe limits (especially below freezing), estimates state of charge and state of health, and reports to the product, often over I²C, SMBus or CAN.

The BMS is often the most safety-critical electronics in a product, and it must work in every state, including deep sleep and long storage, without draining the battery itself.

Where it bites: state-of-charge accuracy shapes the user experience ("it said 20%, then died"); the BMS's own standby current limits shelf life; and safety expectations rise sharply for larger packs.