Designing a BMS with STM32

Designing a Battery Management System (BMS) with STM32 involves defining the BMS requirements, choosing the appropriate microcontroller, designing the hardware, writing the firmware, testing, debugging, and deploying the BMS. This article provides a step-by-step guide to designing a BMS with STM32 and covers topics such as voltage sensing, current sensing, protection circuitry, communication protocols, and firmware development.

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What is BMS?

BMS stands for Battery Management System. It is an electronic system that manages and monitors the performance of rechargeable batteries. BMS is commonly used in electric vehicles, renewable energy systems, and portable electronic devices that use lithium-ion batteries. The primary functions of a BMS include measuring battery voltage, current, and temperature, providing overcharge and over-discharge protection, balancing the charge across individual cells, and reporting the battery’s status to the user. A well-designed BMS can help improve the battery’s performance, extend lifespan, and ensure safe and reliable operation.

Importance of BMS:

The importance of BMS can be understood by looking at the key functions it performs:

1. Monitoring Battery Parameters: BMS continuously monitors the battery voltage, current, and temperature. It allows the user to keep track of the battery’s health, detect potential issues early, and take corrective measures to avoid costly battery failures.
2. Protection against Overcharging and Over-Discharging: BMS protects the battery from overcharging and over-discharging, which can cause permanent damage to the battery and even lead to safety hazards. It disconnects the battery from the charger or the load when the voltage levels exceed the safe limits.
3. Balancing the Charge across Cells: In a multi-cell battery pack, individual cells may discharge at different rates, causing some cells to be over-discharged and others to be overcharged. BMS balances the charge across individual cells, ensuring that each cell operates at the same voltage level and maximizing the battery’s capacity.
4. Extending the Battery Life: By monitoring the battery’s parameters, protecting it from overcharging and over-discharging, and balancing the charge across individual cells, BMS can help extend the battery’s life and improve its performance.
5. Enhancing Safety: BMS enhances the safety of the battery and the system by detecting potential issues early, preventing overcharging and over-discharging, and communicating the battery’s status to the user.

Overall, the importance of BMS lies in its ability to ensure the safe and reliable operation of rechargeable batteries, extend their lifespan, and improve their performance.

What is STM32?

STM32 is a family of microcontrollers developed by STMicroelectronics, a leading semiconductor company based in Switzerland. STM32 microcontrollers are based on ARM Cortex-M processors and are designed for a wide range of applications, including industrial control systems, consumer electronics, and automotive applications.

STM32 microcontrollers offer a range of features, including high processing power, low power consumption, and a wide range of peripherals such as timers, ADCs, DACs, communication interfaces, and memory options. STM32 microcontrollers are also available in various packages and pin configurations, making them suitable for different types of applications.

STM32 microcontrollers are supported by a range of development tools, including IDEs such as STM32CubeIDE and Keil MDK, and are compatible with a range of programming languages such as C, C++, and assembly. STM32 microcontrollers are also widely used in the maker community and are supported by a range of open-source projects and communities.

STM32 microcontrollers are a popular choice for embedded system designers due to their flexibility, performance, and wide range of features.

Designing a BMS using STM32:

Designing a Battery Management System (BMS) using STM32 involves the following steps:

1. Define the BMS requirements:
   • Battery type, voltage, and capacity
   • Number of cells in the battery pack
   • Desired charging and discharging currents
   • Desired safety features such as overcharge, over-discharge, and overcurrent protection
   • Communication protocol requirements
2. Choose the STM32 microcontroller based on the BMS requirements:
   • The STM32 family has a range of microcontrollers suitable for different BMS applications.
   • Consider the number of I/Os, ADC channels, and communication interfaces needed.
3. Design the BMS hardware:
   • Use the STM32 microcontroller as the brain of the BMS.
   • Add voltage sensing circuitry to measure the battery voltage and current sensing circuitry to measure the charging and discharging currents.
   • Include protection circuitry such as overcharge and over-discharge protection circuits, overcurrent protection, and short-circuit protection.
   • Use a communication interface such as CAN, UART, or I2C to communicate with the battery management system.
4. Write the firmware:
   • Develop the firmware for the STM32 microcontroller using an Integrated Development Environment (IDE) such as STM32CubeIDE.
   • Implement the battery charging algorithm based on the battery type, capacity, and desired charging rate.
   • Implement the safety features such as overcharge, over-discharge, and overcurrent protection.
   • Implement the communication protocol for interfacing with the battery management system.
5. Test and debug:
   • Test the BMS hardware and firmware by connecting it to a battery pack.
   • Verify that the BMS is correctly measuring the battery voltage and current and providing the necessary protection and communication functions.
   • Debug any issues found during testing.
6. Deploy the BMS:
   • Deploy the BMS in the final application, such as an electric vehicle, solar energy storage system, or portable electronic device.
   • Monitor the BMS performance and make any necessary adjustments or updates to the firmware.

Example:

Here’s an example code for a basic BMS with STM32 for a 3-cell battery pack:

#include "stm32f4xx.h"

#define VOLTAGE_1_PIN   GPIO_Pin_0
#define VOLTAGE_2_PIN   GPIO_Pin_1
#define VOLTAGE_3_PIN   GPIO_Pin_2
#define CURRENT_PIN     GPIO_Pin_3
#define TEMPERATURE_PIN GPIO_Pin_4

#define OVER_VOLTAGE_THRESHOLD   4.2 // in volts
#define UNDER_VOLTAGE_THRESHOLD  3.0 // in volts
#define MAX_CURRENT              10  // in amps
#define MAX_TEMPERATURE          50  // in degrees Celsius

int main(void)
{
    // Initialize GPIO pins for voltage, current, and temperature sensing
    RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOA, ENABLE);
    GPIO_InitTypeDef gpio;
    gpio.GPIO_Mode = GPIO_Mode_AN;
    gpio.GPIO_Pin = VOLTAGE_1_PIN | VOLTAGE_2_PIN | VOLTAGE_3_PIN | CURRENT_PIN | TEMPERATURE_PIN;
    GPIO_Init(GPIOA, &gpio);
    
    // Initialize ADC for voltage, current, and temperature sensing
    RCC_APB2PeriphClockCmd(RCC_APB2Periph_ADC1, ENABLE);
    ADC_InitTypeDef adc;
    adc.ADC_ContinuousConvMode = ENABLE;
    adc.ADC_DataAlign = ADC_DataAlign_Right;
    adc.ADC_ExternalTrigConv = ADC_ExternalTrigConv_T1_CC1;
    adc.ADC_ExternalTrigConvEdge = ADC_ExternalTrigConvEdge_None;
    adc.ADC_NbrOfConversion = 5;
    adc.ADC_Resolution = ADC_Resolution_12b;
    ADC_Init(ADC1, &adc);
    
    // Configure ADC channels for voltage, current, and temperature sensing
    ADC_RegularChannelConfig(ADC1, ADC_Channel_0, 1, ADC_SampleTime_84Cycles);
    ADC_RegularChannelConfig(ADC1, ADC_Channel_1, 2, ADC_SampleTime_84Cycles);
    ADC_RegularChannelConfig(ADC1, ADC_Channel_2, 3, ADC_SampleTime_84Cycles);
    ADC_RegularChannelConfig(ADC1, ADC_Channel_3, 4, ADC_SampleTime_84Cycles);
    ADC_RegularChannelConfig(ADC1, ADC_Channel_4, 5, ADC_SampleTime_84Cycles);
    
    // Enable ADC and start conversion
    ADC_Cmd(ADC1, ENABLE);
    ADC_SoftwareStartConv(ADC1);
    
    // Main loop
    while (1)
    {
        // Read voltage levels of individual cells
        uint16_t voltage1 = ADC_GetConversionValue(ADC1);
        uint16_t voltage2 = ADC_GetConversionValue(ADC1);
        uint16_t voltage3 = ADC_GetConversionValue(ADC1);
        
        // Calculate battery voltage and check for overvoltage or undervoltage
        float batteryVoltage = (float)(voltage1 + voltage2 + voltage3) / 3.0;
        if (batteryVoltage > OVER_VOLTAGE_THRESHOLD)
        {
            // Implement overvoltage protection
            // ...
        }
        else if (batteryVoltage < UNDER_VOLTAGE_THRESHOLD)
        {
            // Implement undervoltage protection
            // ...
        }
        
        // Read current and temperature levels
        uint16_t current = ADC_GetConversionValue(ADC1);
        uint16_t temperature = ADC_GetConversionValue(ADC1);
        
        // Check for overcurrent or overtemperature
        if (current > MAX_CURRENT)
       {
         // Implement overcurrent protection
         // ...
       }
       if (temperature > MAX_TEMPERATURE)
       {
         // Implement overtemperature protection
         // ...
        }
        // Wait for next ADC conversion
         while (ADC_GetFlagStatus(ADC1, ADC_FLAG_EOC) == RESET);
         ADC_ClearFlag(ADC1, ADC_FLAG_EOC);
     }
   }

Note:

This code initializes the necessary GPIO pins and ADC channels for voltage, current, and temperature sensing. It then reads the voltage levels of each cell in the battery pack, calculates the battery voltage, and checks for overvoltage and under-voltage conditions. It also reads the current and temperature levels and checks for overcurrent and overtemperature conditions. Finally, it waits for the next ADC conversion to occur.

Conclusion:

Note that this code is just a basic example and does not implement any actual protection mechanisms for overvoltage, under-voltage, overcurrent, or overtemperature conditions. These protection mechanisms would need to be implemented separately based on the specific requirements of the BMS application.

Additionally, the code assumes that the ADC channels are configured for single-ended inputs with a reference voltage of 3.3 volts. If different settings are required, these should be adjusted accordingly.

It’s worth noting that this example code only reads the voltage levels of each cell in the battery pack. In a real-world BMS application, additional features such as balancing, state-of-charge estimation, and communication with external devices would also need to be implemented.

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