How Battery Management Systems Work: A Deep Dive

Brief recap of BMS importance The Battery Management System (BMS) is a critical component in modern energy storage solutions, particularly in electric vehicles ...

Jun 13,2025 | Debra

Brief recap of BMS importance

The Battery Management System (BMS) is a critical component in modern energy storage solutions, particularly in electric vehicles (EVs). A BMS ensures the safe, efficient, and reliable operation of battery packs by monitoring and controlling various parameters such as voltage, current, temperature, and state of charge (SOC). Without a BMS, batteries would be prone to overcharging, overheating, and premature failure, which could lead to safety hazards and reduced performance. In Hong Kong, where the adoption of EVs is growing rapidly, the role of BMS in maintaining battery health and extending lifespan cannot be overstated. According to recent data, Hong Kong had over 20,000 registered EVs in 2022, highlighting the increasing demand for advanced BMS technologies. bms meaning battery

Scope of this article: a deeper technical dive

This article aims to provide a comprehensive technical exploration of how Battery Management Systems (BMS) work, focusing on their key functions, cell balancing techniques, and communication protocols. By delving into the intricacies of BMS, we hope to shed light on the sophisticated engineering behind these systems and their pivotal role in electric vehicles. Whether you're an engineer, a student, or simply an EV enthusiast, this deep dive will equip you with a thorough understanding of BMS technology.

Voltage Monitoring: Accuracy and Precision

Voltage monitoring is one of the most fundamental functions of a BMS. It involves measuring the voltage of individual cells and the entire battery pack to ensure they operate within safe limits. Accurate voltage measurement is crucial because even minor deviations can lead to overcharging or over-discharging, both of which can damage the battery. In electric vehicles, where high-voltage battery packs are common, precision is paramount. Modern BMSs use high-resolution analog-to-digital converters (ADCs) to achieve voltage measurements with an accuracy of ±1mV. Calibration techniques, such as periodic zero-offset correction, are employed to maintain measurement accuracy over time.

How voltage is measured in individual cells and packs

Voltage measurement in a BMS typically involves connecting each cell to a voltage sensing circuit. These circuits are designed to handle the high voltages found in EV battery packs, which can range from 200V to 800V. The BMS continuously scans each cell's voltage and compares it against predefined thresholds. If any cell exceeds these thresholds, the BMS takes corrective action, such as disconnecting the load or initiating cell balancing.

Impact of measurement errors

Measurement errors in voltage monitoring can have severe consequences. For instance, an underestimated voltage reading might cause the BMS to overcharge a cell, leading to thermal runaway—a dangerous condition where the battery overheats and potentially catches fire. Conversely, an overestimated reading might result in premature shutdown, reducing the vehicle's range. To mitigate these risks, BMSs incorporate redundant sensing paths and error-checking algorithms.

Calibration techniques

Calibration is essential to maintain the accuracy of voltage measurements over the battery's lifespan. Common techniques include:

• Zero-offset calibration: Adjusts for any DC bias in the measurement circuit.
• Gain calibration: Ensures the scaling factor between the actual voltage and the measured value is correct.
• Temperature compensation: Accounts for variations in component performance due to temperature changes.

Current Sensing: Measuring Inflow and Outflow

Current sensing is another critical function of a BMS, as it directly impacts the estimation of the battery's State of Charge (SOC). By measuring the current flowing into and out of the battery, the BMS can calculate the amount of energy stored or consumed. In electric vehicles, current sensing is particularly important for regenerative braking, where energy is recovered during deceleration and fed back into the battery.

Methods for current sensing

There are several methods for current sensing in BMS applications:

• Shunt resistors: A low-resistance resistor placed in series with the battery. The voltage drop across the resistor is proportional to the current.
• Hall effect sensors: These sensors measure the magnetic field generated by the current, providing galvanic isolation and high accuracy.
• Current transformers: Used for AC current measurement, though less common in DC applications like EVs.

Impact of current measurement on State of Charge (SOC) estimation

Accurate current measurement is vital for SOC estimation, which is typically calculated using the Coulomb counting method. This method integrates the current over time to determine the amount of charge added or removed from the battery. Any error in current measurement will propagate into the SOC estimate, leading to incorrect range predictions in EVs. Advanced BMSs use sensor fusion techniques, combining data from multiple sensors, to improve SOC estimation accuracy.

Temperature Management: Keeping Batteries Cool (or Warm)

Temperature management is a key aspect of BMS functionality, as battery performance and lifespan are highly temperature-dependent. Excessive heat can accelerate degradation, while extreme cold can reduce efficiency. In Hong Kong's subtropical climate, where temperatures can soar above 35°C, effective thermal management is essential for EV batteries.

Temperature sensors and their placement

BMSs use thermistors or digital temperature sensors to monitor battery temperature. These sensors are strategically placed at hotspots within the battery pack, such as near high-current connectors or between cells. The data from these sensors is used to trigger cooling or heating systems as needed. battery management system bms

Thermal management strategies

Common thermal management strategies include:

• Air cooling: Uses fans to circulate air around the battery pack.
• Liquid cooling: Circulates coolant through channels in the battery pack for more efficient heat dissipation.
• Phase-change materials: Absorb heat during phase transitions, providing passive cooling.

State of Charge (SOC) Estimation

SOC estimation is one of the most challenging aspects of BMS design. It refers to the remaining charge in the battery as a percentage of its full capacity. Accurate SOC estimation is crucial for EVs, as it directly impacts the driver's range prediction.

Coulomb counting method

The Coulomb counting method involves integrating the current over time to estimate SOC. While straightforward, this method is prone to errors due to sensor inaccuracies and battery aging. To compensate, BMSs often reset the SOC based on voltage measurements when the battery is fully charged or discharged.

Voltage-based method

The voltage-based method estimates SOC by correlating the battery's open-circuit voltage (OCV) with its charge state. This method is less accurate under load but can be used to calibrate the Coulomb counting method during periods of rest.

Kalman filtering for SOC estimation

Advanced BMSs use Kalman filters to combine data from multiple sources, such as current, voltage, and temperature sensors, to improve SOC estimation. This approach accounts for measurement noise and battery nonlinearities, providing a more accurate and reliable SOC estimate.

State of Health (SOH) Estimation

SOH estimation is another critical BMS function, as it indicates the battery's overall condition and remaining useful life. SOH is typically expressed as a percentage of the battery's original capacity or internal resistance. battery management system in electric vehicles

Capacity fade analysis

Capacity fade refers to the gradual loss of battery capacity over time. BMSs track capacity fade by comparing the actual charge delivered during discharge cycles with the battery's rated capacity. This data is used to predict the battery's remaining lifespan.

Impedance spectroscopy

Impedance spectroscopy measures the battery's internal resistance at different frequencies. An increase in internal resistance is a strong indicator of aging and can be used to estimate SOH.

Cell Balancing Techniques Explained

Cell balancing is essential for maintaining uniform charge levels across all cells in a battery pack. Imbalances can lead to reduced capacity and premature failure.

Passive Balancing: Bleeding Excess Charge

Passive balancing dissipates excess energy from higher-charged cells as heat through resistors. While simple and cost-effective, this method is inefficient and generates heat.

Active Balancing: Redistributing Charge

Active balancing transfers energy from higher-charged cells to lower-charged ones, improving efficiency. Common methods include:

• Switched capacitor method: Uses capacitors to transfer charge between cells.
• Inductive balancing method: Uses inductors to transfer energy, offering higher efficiency.

BMS Communication Protocols

BMSs rely on communication protocols to exchange data with other vehicle systems. These protocols ensure seamless integration and real-time monitoring.

CAN Bus: The Industry Standard

The Controller Area Network (CAN) bus is the most widely used protocol in automotive applications. It offers robust, high-speed communication and is capable of connecting multiple devices on a single network.

Modbus RTU

Modbus RTU is a serial communication protocol commonly used in industrial applications. It is simple and reliable but lacks the speed and flexibility of CAN.

I2C and SPI

I2C and SPI are used for short-distance communication within the BMS, such as between microcontrollers and sensors. They offer high-speed data transfer but are limited in range.