EV Battery Management System

Project Overview

Designed and implemented the complete BMS firmware stack for a 96S4P (400V) NMC lithium-ion battery pack used in a series-production electric vehicle. The system handles all safety-critical functions from cell monitoring to CAN communication with the vehicle ECU.

Key Challenges

• ASIL-B compliance — Implemented redundant voltage measurement paths, end-to-end CRC checks on all CAN messages, and software-level diagnostics meeting ISO 26262 Part 6 requirements.
• SoC accuracy under dynamic loads — Pure Coulomb counting drifts under temperature variation and aging. Implemented an Extended Kalman Filter that fuses current integration with OCV measurements during relaxation periods, achieving ±1% accuracy across the full SoC range.
• Cell balancing during charging — Passive balancing with intelligent threshold management — only balances when pack is in the CV phase of charging to avoid wasted energy during driving.

SoC Estimation Algorithm

/* EKF predict step — called every 100ms */
void bms_ekf_predict(EKF_t *ekf, float current_A, float dt_s) {
    /* State: SoC (0.0 to 1.0) */
    ekf->x -= (current_A * dt_s) / PACK_CAPACITY_AS;
    ekf->x  = fclampf(ekf->x, 0.0f, 1.0f);
    ekf->P += ekf->Q;  /* Covariance propagation */
}

/* EKF update step — triggered when current < 1A (relaxed) */
void bms_ekf_update(EKF_t *ekf, float v_terminal) {
    float v_ocv  = ocv_lookup(ekf->x, bms.temp_avg_C);
    float H      = docv_dsoc(ekf->x);   /* Jacobian */
    float S      = H * ekf->P * H + ekf->R;
    float K      = ekf->P * H / S;      /* Kalman gain */
    ekf->x      += K * (v_terminal - v_ocv);
    ekf->P       = (1.0f - K * H) * ekf->P;
}

Architecture

• Hardware — STM32G474 with hardware floating-point, paired with LTC6812 AFE for cell voltage and temperature measurement
• RTOS — FreeRTOS with 5 priority levels: Safety Monitor (highest), Cell Sampling, State Estimation, CAN Communication, Diagnostics
• Communication — CANopen DS301/DS305 with 4 TPDOs for telemetry and 2 RPDOs for control commands
• NVM — Coulomb counter and SoH data stored in internal Flash with wear-leveling across 16 sectors

Test & Validation

• HIL testing with National Instruments cRIO simulating cell voltages and temperatures
• 600+ hours of continuous operation validation on battery pack emulator
• Fault injection testing covering 47 fault modes (OV, UV, OT, OC, wire-off, AFE fault)
• MISRA-C 2012 compliance verified with PC-lint Plus

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