STM32-Based Lithium-Ion Battery Management System Development

In this study, a battery management system (BMS) consisting of 15-cell series has been developed for a 48 V industrial energy storage application.

Monitoring of cell voltages and temperatures, passive balancing of cells, state-of-charge and fault information transmission over CAN bus, and application of layered safety measures for protection against overvoltage, undervoltage, overcurrent, and overtemperature were among the requirements placed before the BMS developers.

The performance targets included: measurement accuracy for cell voltage of ±2 mV, measurement accuracy for temperature of ±1 °C, balancing activation threshold of 50 mV cell-to-cell difference between voltages basic data transmission speed over CAN bus of 500 kbps while using CANopen protocol in the message format.

The BMS also needed to support field calibration of current and voltage channels without firmware recompilation.

The central processing unit was the STM32F446RET6, an ARM Cortex-M4F microcontroller running at 180 MHz with 512 KB Flash and 128 KB SRAM.

This device was selected for its dual CAN controllers (bxCAN), three 12-bit ADCs, multiple timers for PWM and input capture, and sufficient computational headroom for coulomb-counting state-of-charge estimation and protection logic.

2.1 AFE Selection and Architecture

Cell voltage measuring is now performed by a special analog front-end (AFE) which does not use the internal ADC of the STM32 anymore.

In this case, we decided to use the Texas Instruments BQ76940 which not only provides a 14-bit ADC for measuring the cell voltage, but it also has a 16-bit ADC for measuring the pack current, and allows connecting three external NTC thermistors for the temperature measurement.

The BQ76940 allows connecting up to 15 series cells which exactly matches our target configuration.

The AFE communicates with STM32 using I²C with the frequency of 400 kHz and the I²C isolator (ISO1540) provides galvanic isolation between the AFE ground on the battery side and the microcontroller ground.

2.2 Voltage Measurement Chain

Voltages of each individual cell were measured differentially between the terminals of the cell using the RC filter network with one 100 ohm resistor in series and one 100 nF capacitor connected to the negative terminal of the cell forming a low-pass filter with the cut-off frequency of approximately 16 kHz.

This means that the RC filter was able to eliminate switching noise coming from the power electronic part while not introducing any additional settling error to the 1 Hz conversion rate of the AFE.

The internal multiplexer of the BQ76940 sampled all the cells in time of 1 millisecond; the STM32 was able to poll all the AFE registers every 100 milliseconds and get all voltages of 15 cells and compute the total of the pack.

Resolution of the cell's voltage was estimated to be equal to 1 mV per LSB with accuracy of ±2 mV guaranteed during the factory calibration in the temperature range of −40 °C to +85 °C.

A separate voltage divider (100 kΩ/10 kΩ) fed an STM32 ADC channel for pack-level voltage monitoring, providing redundant measurement independent of the AFE. This redundancy was a deliberate safety requirement: if the AFE failed, the STM32 could still detect gross overvoltage or undervoltage and initiate protection.

Three 10 kΩ NTC thermistors were placed at three different locations: one in the center, one at the positive terminal and one at the negative terminal.

The thermistors were wired as a voltage divider having a 10 kΩ precision resistor which has 0.1% tolerance and 25 ppm/°C sensitivity with respect to the AFE's 3.3 V power supply.

The voltage of the divider was measured by BQ76940 internal ADC and the readings were processed by STM32 taking into account Steinhart-Hart equation.

1/T = A + B·ln(R) + C·(ln(R))³

The coefficients A, B, and C have been stored in the Flash memory so that they can be updated later during calibration. The 14-bit ADC of the AFE has provided a temperature resolution of around 0.1 °C, which is above the specified maximum accuracy limit of ±1 °C.

Additionally, there was an overtemperature comparator in the BQ76940 chip that guaranteed the redundancy of the system. If any voltage across the thermistor goes below a programmed threshold value of 60 °C, the ALERT pin of the AFE is already pulled high, thus, the AFE sends a signal to the STM32 microcontroller, which will start processing actions in response to it.

The current was determined using a 1 mΩ shunt resistor that was attached to the negative line of the battery pack.

The differential voltage across the shunt was amplified by the INA240 current-sense amplifier, which has a gain of 50, thereby giving an output of 0-3.3 V with a current range of ±66 A.

The bandwidth and the gain accuracy of the INA240 ensured accurate measurement of the current during steady and transient conditions.

The output of the amplifier was picked by the ADC1 of the STM32 at a rate of once every second.

Simultaneously, the BQ76940's internal 16-bit current ADC provided an independent measurement path with a maximum error of 0.25% after customer end-of-line calibration.

The two measurements were compared in firmware; if they differed by more than 5%, a sensor fault was logged and the BMS transitioned to a degraded mode using only the AFE current reading.

5.1 Passive Balancing Circuit

Passive balancing was implemented using the BQ76940's integrated balancing FETs, which selectively connected an external bleed resistor across cells with excess charge. The external resistor value was 33 Ω, giving a balancing current of approximately 100 mA per active cell at 3.3 V. This current was sufficient to equalize a 50 mV deviation in approximately 30 minutes for a 10 Ah cell.

5.2 Balancing Control Logic

The balancing logic executed in the control loop with 100 ms intervals. It calculated the mean value of the voltage of all cells, not subjected to balancing, identified those which had the voltage above the mean level by over 50 mV.

The balancing process was acceptable only when the battery pack was safe: there were no existing faults, the battery pack was at temperature in the range of 10 °C to 40 °C, and the current was under 5 A.

The BQ76940's balance FETs were enabled through I²C register writes, with a maximum of four cells balanced simultaneously to limit thermal dissipation in the AFE package. Adjacent-cell balancing was prohibited to prevent unintended current paths through the measurement multiplexer.

6.1 Physical Layer

The CAN communication utilized the bxCAN peripheral of the STM32, operational in its standard mode at a speed of 500 kbps, with a sampling point of 87.5%.

The physical level included an isolated transceiver (ADM3053), providing a voltage isolation of 2.5 kV RMS between the Battery Management System (BMS) and the vehicle/industrial CAN bus.

The bus side included a termination resistor of 120 Ω, which was connected permanently, along with fail-safe biasing resistors (390 Ω to 5 V and 1.5 kΩ to the Earth) for proper recessive levels of the bus line in case of disconnection from the bus.

6.2 Message Profile

The BMS transmitted three periodic CAN messages at 10 Hz, following a profile compatible with CANopen process data objects (PDOs):

The arrangement of the message is such that it is easy to analyze by a vehicle controller or energy management system.

The voltage in the pack is transmitted in 0.1 V, the current in 0.1 A, and the state of charge in one byte (0-100%). The voltage across the cells utilizes 12-bit packing and thus is able to fit all 15 values into 30 bytes and four CAN frames.

6.3 Fault Reporting

Faults were sent via event-driven messages on CAN ID 0x480, which specified the type of a fault by using a bitfield (cell overvoltage, cell undervoltage, overcurrent, overtemperature, AFE communication failure or sensor disagreement). In addition, the number of the cell that was affected and its measured value were also included.

7.1 Protection Hierarchy

Safety protection was implemented at three levels:

Hardware level:

The BQ76940's internal comparators monitored cell voltages and temperatures independently of the STM32. If any threshold was exceeded, the AFE asserted its ALERT pin and could directly drive external protection MOSFETs via its CHG and DSG outputs.

Firmware level:

The STM32's control loop checked all measured values against configurable thresholds every 100 ms. If a threshold was breached, the firmware commanded the AFE to open the protection MOSFETs and set a fault flag.

Comparator level:

The STM32's analog comparators (COMP1/COMP2) monitored the current-sense amplifier output and pack voltage divider, providing hardware-speed response (under 1 µs) for overcurrent and short-circuit events.

7.2 Threshold Configuration

The BQ76940's internal Non-volatile memory (NVM) was used for safety thresholds storage, which eliminates the need for reprogramming every time the device wakes up.

The standard threshold values are cell overvoltage of 4.25 volts, cell undervoltage of 2.50 volts, pack overcurrent of 50 A while discharging and 30 A while charging, and over-temperature of 60 °C.

The values provided are suitable for the LiFePO₄ technology and they can be modified using a CAN configuration message.

7.3 Recovery Logic

Following the occurrence of the protective event, the BMS entered the latched fault state. The recovery process called for the absence of the fault condition for 30 seconds in a row as well as a manual or CAN-commanded clearance. In the case of overvoltage and over-temperature faults, automatic recovery was forbidden so that these situations would not enter unsafe cycling.

8.1 Voltage Channel Calibration

Cell voltage channels were calibrated at end-of-line using a precision voltage source (Fluke 5520A) applied sequentially to each cell input. The STM32 recorded the AFE reading at three points (2.5 V, 3.3 V, 4.2 V) and computed a linear gain and offset correction stored in Flash. This reduced measurement error from the AFE's uncalibrated ±5 mV to the target ±2 mV.

8.2 Current Channel Calibration

In the present calibration, a programmable electronic load was utilized for the adjustment of the desired currents (±5 A, ±20 A, ±50 A) while the output of the INA240 was measured. Calibration was therefore performed on the basis of two points (the offset and the gain).

The current ADC of the BQ76940 was calibrated at the same time in accordance with the internal calibration registers, and the achieved accuracy of calibration was 0.25% according to the specifications of the manufacturer.

8.3 Temperature Channel Calibration

The temperature calibration was done using a highly accurate thermistor simulator (Fluke 724) to provide known resistance values for the various temperature points (0 °C, 25 °C, and 60 °C).

The values of the Steinhart-Hart coefficients were then recalculated based on the actual readings from the AFE. After that, the set coefficients were recorded in Flash memory.

During the validation of the procedure, it was verified that the value of the calibrated system was within ±0.5 °C of the reference for the entire range.

Battery management system was evaluated on 15-cell lithium iron phosphate battery pack (48 V, 20 Ah). Testing involved using battery cycler to confirm voltage measurement accuracy on all 15 channels; maximum deviation determined was 1.8 mV.

The temperature accuracy was ±0.7 °C. Balancing test revealed that the initial deviation of 120 mV was brought within 45 minutes to under 20 mV.

The Vector CANoe interface validated CAN communication. Message timing, message data packing, and fault bitfield encoding all succeeded.

Overvoltage, undervoltage, overcurrent, and overtemperature protection tests confirmed detection and clearing thresholds. A 200-hour 1C charge/discharge endurance test showed no false trigger from protection and ensured SoC estimation error well below 3%.

The BMS that is based on STM32 met all of the requirements concerning communication, accuracy, and safety for the application that was intended to be accomplished with its help.

The main conclusions made by the engineers are the importance of the dedicated AFE for precise measuring of cells, the need for independent redundant measurement paths for parameters that were of critical importance, and the significance of the final stage of the calibration applied for production achieving accuracy proved by the data sheet.

The architecture of protection built with the help of layers such as hardware, firmware, and comparator prevented having a single point of failure, which could influence the safety of the batteries, and therefore the system is successfully used in the production energy storage unit.