BMS Architecture and Core Functions
The Battery Management System (BMS) is the microcontroller-based intelligence layer that sits between the battery pack and the rest of the vehicle's electrical system. A modern EV BMS executes six primary functions simultaneously. First, protection: it monitors every cell's voltage (typically ±1 mV resolution), temperature, and current, triggering contactors to disconnect the pack if any parameter exceeds safety limits — overvoltage above ~4.2V/cell, undervoltage below ~2.5V/cell, overtemperature above 45–60°C, or overcurrent during a short circuit. Second, State of Charge (SoC) estimation: using coulomb counting (integrating current over time) combined with open-circuit voltage lookup tables and Kalman filter algorithms to achieve ±1–2% accuracy despite sensor noise. Third, State of Health (SoH) estimation: tracking capacity fade over thousands of cycles using impedance spectroscopy or incremental capacity analysis. Fourth, cell balancing. Fifth, thermal management coordination. Sixth, CAN bus communication to the vehicle's main ECU and to charging equipment via ISO 15118 or CHAdeMO protocols. Tesla's BMS runs on ARM Cortex processors monitoring up to 7,104 individual 18650 cells (in older Model S packs) or 828 4680 cells (in Cybertruck), sampling every cell voltage every 100 milliseconds.