Design and build a full-featured BMS for a 7S Li-ion battery pack with cell balancing, SOC estimation, temperature protection, and CAN bus telemetry.
Design and build a full-featured BMS for a 7S Li-ion battery pack with cell balancing, SOC estimation, temperature protection, and CAN bus telemetry. This comprehensive guide covers everything from design through implementation, testing, and deployment.
Li-ion batteries require strict control to prevent: overcharge (>4.20V per cell → electrolyte decomposition, thermal runaway), over-discharge (<2.5V → copper dissolution, permanent capacity loss), overcurrent (short circuit → rapid self-heating → thermal runaway → fire), over-temperature (>60°C → accelerated aging, thermal runaway risk). BMS functions: Cell voltage monitoring (each cell individually), Current monitoring (charge + discharge), Temperature monitoring (cells + PCB), Protection switches (disconnect pack on fault), Cell balancing (equalize cell SOC), SOC estimation (remaining capacity), Communication (telemetry to host).
10 components required for this project.
| # | Component | Purpose | Qty |
|---|---|---|---|
| 1 | BQ76940 Battery Monitor IC (15S capable) | Cell voltage and temperature monitoring | x1 |
| 2 | BQ76200 High-Side FET driver | Charge/discharge MOSFET switching | x1 |
| 3 | N-channel MOSFETs (100V, 30A) × 2 | Pack charge and discharge switches | x2 |
| 4 | Passive balancing resistors (2Ω, 1W per cell) | Cell voltage equalization | x7 |
| 5 | NTC thermistors (10kΩ) × 4 | Cell and PCB temperature sensing | x4 |
| 6 | STM32G0 MCU (BMS host) | SOC algorithm and CAN telemetry | x1 |
| 7 | SN65HVD230 CAN transceiver | CAN bus communication | x1 |
| 8 | Current sensor (ACS758, ±50A Hall effect) | Pack charge/discharge current | x1 |
| 9 | Li-ion cells (Samsung 21700, 5Ah) × 7 | Battery cells (7S configuration) | x7 |
| 10 | Coulomb counter (LTC4150) | Integrated current-time for SOC | x1 |
Follow these 4 steps carefully.
Li-ion batteries require strict control to prevent: overcharge (>4.20V per cell → electrolyte decomposition, thermal runaway), over-discharge (<2.5V → copper dissolution, permanent capacity loss), overcurrent (short circuit → rapid self-heating → thermal runaway → fire), over-temperature (>60°C → accelerated aging, thermal runaway risk). BMS functions: Cell voltage monitoring (each cell individually), Current monitoring (charge + discharge), Temperature monitoring (cells + PCB), Protection switches (disconnect pack on fault), Cell balancing (equalize cell SOC), SOC estimation (remaining capacity), Communication (telemetry to host).
BQ76940 monitors up to 15 series cells. Each cell connected to VC+ and VC- pins. Integrated ADC measures cell voltages with ±1mV accuracy. Temperature inputs: NTC thermistors via TS1–TS3 pins. Communication: I2C to MCU at 400kHz. Registers: CELLVOLTAGE1–15 (16-bit each), SYS_STAT (fault flags: OV, UV, OCD, SCD, OVRD_ALERT). Alerts: OV (over voltage), UV (under voltage) hardware comparators with programmable thresholds (via OV_TRIP, UV_TRIP registers). SCD (Short Circuit in Discharge): hardware latch within 70µs — far faster than MCU response.
SOC estimation methods: Coulomb Counting (integrate current over time: SOC = SOC_initial - ∫I×dt / capacity). Accurate when reset is possible, accumulates drift over time. Voltage-Based: OCV (Open Circuit Voltage) vs SOC table lookup. Accurate at rest (after 2h equilibration), useless during load. Kalman Filter: combines Coulomb Counting with OCV correction, accounts for temperature and aging effects. Battery model: equivalent circuit (V_oc + R_internal + RC pairs for transient response). Extended Kalman Filter is industry standard for EV BMS.
Passive balancing: excess charge in high cells dissipated as heat through resistor. Simple, reliable, cheap. Energy wasted (efficiency loss). Bypass controlled by BMS: when cell voltage > (pack_average + 10mV), enable balancing resistor. Continue until within 5mV. Only balance near full SOC (cells most divergent at top). Active balancing: transfer energy from high cells to low cells using DCDC converters. High efficiency (80–90%), complex, expensive. Used in premium EV packs. For this project: passive balancing via BQ76940's integrated balancing switches (with external resistors).
Core code for bms_soc.c:
// BMS SOC Estimation using Coulomb Counting + OCV correction // Runs on STM32G0, communicates with BQ76940 via I2C #include <stdint.h> #include <math.h> #define CELL_COUNT 7 #define CAPACITY_MAH 5000.0f // 5Ah cell #define COULOMB_PERIOD_MS 100 // SOC update every 100ms // OCV-SOC lookup table for Samsung 21700 at 25°C // [SOC%]: [OCV in mV] static const float OCV_TABLE[11][2] = { {0, 2500}, {10, 3400}, {20, 3550}, {30, 3650}, {40, 3700}, {50, 3730}, {60, 3760}, {70, 3800}, {80, 3870}, {90, 3960}, {100, 4200} }; float soc_percent = 50.0f; // Initial SOC estimate float accumulated_mah = 0; float ocv_to_soc(float ocv_mv) { for(int i = 0; i < 10; i++) { if(ocv_mv >= OCV_TABLE[i][1] && ocv_mv <= OCV_TABLE[i+1][1]) { float t = (ocv_mv - OCV_TABLE[i][1]) / (OCV_TABLE[i+1][1] - OCV_TABLE[i][1]); return OCV_TABLE[i][0] + t * (OCV_TABLE[i+1][0] - OCV_TABLE[i][0]); } } return (ocv_mv < OCV_TABLE[0][1]) ? 0 : 100; } void bms_update_soc(float current_mA, float rest_voltage_mv, bool at_rest) { // Coulomb counting float delta_mah = current_mA * (COULOMB_PERIOD_MS / 3600000.0f); accumulated_mah += delta_mah; soc_percent -= (delta_mah / CAPACITY_MAH) * 100.0f; soc_percent = fmaxf(0, fminf(100, soc_percent)); // OCV correction at rest (correct coulomb counting drift) if(at_rest) { float soc_from_ocv = ocv_to_soc(rest_voltage_mv); // Kalman-like update: weight OCV heavily at rest soc_percent = 0.3f * soc_percent + 0.7f * soc_from_ocv; } }Test Battery Management System (BMS) by verifying each subsystem individually before full integration.
Verify power voltages, check ground connections, use serial monitor for debug.
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