What is a Battery Management System (BMS)? How Are Electric Vehicle Batteries Controlled?

What is a Battery Management System (BMS)?

A Battery Management System (BMS) is an electronic control system that monitors the operating status of rechargeable battery packs, oversees safety limits, and manages the battery's usable capacity and performance.

In electric vehicles, high-voltage battery packs are formed by bringing together hundreds—and in some battery architectures, thousands—of cells. It is impossible for all these cells to maintain identical voltage, temperature, capacity, and aging characteristics. Minor differences arising from manufacturing tolerances and operating conditions can lead to performance loss, capacity imbalance, and safety issues over time.

The primary duty of a BMS is to continuously monitor the cells in the battery pack and the general operating conditions of the system, ensuring that the battery operates within designated safe limits.

The system tracks cell voltages, pack current, temperatures, and other operating parameters. Using the collected data, values such as the battery's state of charge, state of health, and the power it can safely supply or accept are also calculated.

Why is a BMS Necessary in Electric Vehicles?

Lithium-ion batteries are widely used in modern electric vehicles due to their high energy density, high efficiency, and long cycle life. However, for these cells to operate safely, parameters such as voltage, current, and temperature must be kept within specified limits.

Overcharging or over-discharging a cell can accelerate capacity loss and reduce the cell's lifespan. High temperatures can also accelerate chemical aging processes within the battery. Under more severe fault conditions, thermal events that compromise cell safety may occur.

Therefore, the BMS continuously monitors the operating state of the battery pack and ensures that charging or discharging power is limited when necessary.

For example, if any cell approaches its maximum allowable voltage level, the BMS can request lower power from the charging system. If the battery temperature rises, power limits can be adjusted, or higher cooling capacity can be requested from the thermal management system.

Thus, the BMS forms a critical control and safety layer between the electric vehicle's high-voltage battery pack and its other electronic and electrical systems.

Which Parameters Does a BMS Measure?

To determine the real-time operating status of the battery pack, the Battery Management System collects data from various sensors and measurement circuits.

One of the most important measurements is cell voltage. By monitoring the voltages of individual cells or specific cell groups within the battery pack, cells at risk of overcharging or over-discharging can be identified.

The current entering and leaving the battery pack is also measured using precise current sensors. This data plays an important role in both overcurrent protection and calculating the battery's state of charge.

Another critical parameter is temperature. Sensors placed at various points across the battery modules monitor the thermal status of the cells and modules.

The primary parameters monitored in modern BMS systems include:

• Cell voltages
• Total battery pack voltage
• Charge and discharge current
• Cell and module temperatures
• Electrical insulation status
• Status of high-voltage contactors
• Battery pack fault and safety conditions
• Certain cooling circuit parameters, depending on the system

These measurements are essential not only for battery safety but also for calculating usable capacity and power limits.

What is SOC? How is Battery State of Charge Calculated?

The battery percentage displayed to the driver is not a value directly measured by a single sensor.

This value, calculated by the BMS, is called the State of Charge (SOC).

SOC represents the approximate level of energy available in the battery. 100% SOC corresponds to the upper operational limit defined by the manufacturer, while 0% corresponds to the lower limit. Therefore, the 0% and 100% values on the vehicle display do not necessarily match the absolute electrochemical limits of the cell.

Different methods can be used together to calculate SOC.

One such method is coulomb counting, which integrates the current entering and leaving the battery over time. However, relying solely on this method may not be sufficient due to the accumulation of small measurement errors from current sensors over time.

For this reason, modern BMS algorithms evaluate current, cell voltage, temperature, and information derived from mathematical models of the battery simultaneously.

In more advanced systems, state estimation methods such as Kalman filtering or other model-based algorithms may also be utilized.

The accuracy of the SOC estimation is vital not only for the battery percentage shown to the driver but also for remaining range calculation and power management.

What is SOH? How is Battery Aging Tracked?

The usable capacity and performance of batteries do not remain constant throughout their lifespan. Charge-discharge cycles, high temperatures, high currents, prolonged periods at high charge levels, and the natural effects of time cause cell aging.

An important indicator tracked or estimated by the BMS is the State of Health (SOH).

SOH expresses the current capacity and performance level of the battery compared with its condition when new.

Calculating SOH does not rely on a single parameter. Usable capacity, internal resistance of cells, power delivery capability, and long-term aging behaviors can be evaluated together.

For instance, a battery that could store a specific amount of usable energy when new beginning to offer lower capacity over time is one of the primary signs of battery aging.

SOH estimation is important not only for evaluating long-term vehicle performance but also for maintenance planning, warranty analysis, and second-life battery applications.

What is Cell Balancing?

Even if the cells in a battery pack belong to the same model and production batch, they may not exhibit identical electrical characteristics.

Due to manufacturing tolerances, temperature variations, and cells aging at different rates over time, some cells may reach maximum or minimum voltage levels earlier than others.

For example, if one cell reaches maximum voltage sooner than the rest, the charging process for the entire battery pack may be restricted because of that single cell. Similarly, a cell reaching minimum voltage earlier during discharge can reduce the overall usable capacity of the battery.

To minimize these discrepancies, cell balancing methods are used in BMS systems.

One of the most common methods is passive cell balancing. In this approach, a small amount of energy from cells with higher charge levels is dissipated as heat through resistors, reducing the difference between cells.

In more complex systems, active cell balancing methods may be employed. In these systems, energy can be transferred from cells with higher charge levels to those with lower levels, or to another part of the battery system.

Active balancing can be more energy-efficient, but it requires additional power electronics components and more complex control algorithms.

How Does a BMS Protect the Battery?

The Battery Management System continuously executes various protective functions to prevent the battery pack from operating outside the safe operating area specified by the manufacturer.

Key protection functions performed by a BMS include:

• Overvoltage protection for cells
• Undervoltage protection for cells
• Overcurrent protection
• Detection of short circuits or abnormal current conditions
• Overtemperature protection
• Charge restriction at low temperatures
• Monitoring of electrical insulation faults
• Supervision of contactor and high-voltage system faults

When a critical condition is detected, the BMS can notify other control units in the vehicle that the allowed power level must be reduced.

In the event of a more severe fault, the main contactors that isolate the battery pack from the vehicle's high-voltage system can be opened.

Therefore, the BMS is not merely a control system that manages battery performance, but also a fundamental component of the electric vehicle's high-voltage safety architecture.

How Do the BMS and Thermal Management System Work Together?

Battery temperature directly affects the performance, fast-charging capability, power delivery, and lifespan of lithium-ion cells.

At very low temperatures, the battery's power delivery and charge acceptance capability decrease. Because charging lithium-ion cells with high currents at low temperatures can lead to undesirable electrochemical processes, the BMS may significantly limit charging power.

High temperatures, on the other hand, can accelerate cell aging and increase safety risks.

For this reason, the BMS continuously collects data from temperature sensors placed throughout the battery pack.

When the temperature moves outside the target range, the BMS or the vehicle's higher-level energy management system can request cooling or heating in coordination with the thermal management system.

In electric vehicles, battery thermal management can be implemented using air cooling, liquid cooling, or other heat-transfer systems.

In advanced electric vehicles, the battery, electric motor, inverter, and cabin climate control systems can operate as interconnected parts of an integrated thermal management system.

How Does the BMS Communicate with Other Vehicle Systems?

In modern electric vehicles, the BMS is not an independently operating electronic system.

The battery management system continuously exchanges data with the vehicle control unit (VCU), inverter, charging system, thermal management system, and other electronic control units.

In this communication, various in-vehicle communication technologies are utilized, primarily Controller Area Network (CAN), which is widely used in the automotive sector.

The BMS transmits data to other control units, including SOC, SOH, battery temperature, fault information, and the maximum power levels the battery can safely supply or accept.

For example, if the battery temperature is unsuitable for high power demands, the BMS can restrict the available discharge power through the vehicle's energy and drive control architecture.

Similarly, during fast charging, the maximum charging power the battery can accept is determined based on cell temperature, SOC, voltage levels, and the current state of the battery.

Thanks to this communication, the battery, motor drive, charging system, and thermal management system can function as a single, coordinated energy system.

What is the Hardware Structure of a BMS?

The BMS architecture used in electric vehicles can vary depending on the size of the battery pack, the number of cells, and the manufacturer's system design.

In smaller battery systems, a centralized BMS architecture may be used. In this structure, most measurement and control functions are executed within a single electronic control unit.

In the large battery packs used in electric cars, modular or distributed architectures are common.

Cell monitoring electronic circuits placed on the battery modules measure cell voltages and temperatures. The collected data is transferred to the higher-level main BMS control unit.

The main control unit evaluates the overall status of the battery pack, calculates values such as SOC and SOH, determines power limits, and communicates with the vehicle's other electronic control systems.

This structure makes measurement wiring, electronic hardware, and data flow far more manageable in high-voltage battery packs consisting of hundreds of cells.

How Does a BMS Affect Electric Vehicle Performance?

The performance of an electric vehicle does not depend solely on the chemical properties of the battery cells or the maximum power of the motor.

The limits calculated by the BMS significantly determine how much power the battery can supply under current conditions and how much energy it can safely accept.

These limits can affect the vehicle's acceleration performance, fast-charging capability, and the amount of energy that can be recovered during regenerative braking.

For instance, when the battery is very cold, the BMS may reduce the charge power accepted during regenerative braking to protect the cells.

Similarly, when the battery reaches a very high SOC level, regenerative braking capacity may be limited because there is insufficient remaining capacity to store the additional energy.

Likewise, at high battery temperatures, discharge power can be limited, temporarily reducing the maximum power the drive system can deliver.

Therefore, behind many performance variations observed by drivers in electric vehicles lies the real-time energy and safety management performed by the BMS.

The Future of Battery Management Systems

As the energy capacity, fast-charging power, and system complexity of electric vehicle batteries increase, the role of BMS technologies continues to expand.

Next-generation BMS systems utilize more advanced battery models, more precise state estimation algorithms, and microcontrollers with greater computing power.

Wireless BMS (wBMS) architectures are also among the key approaches being developed. In these systems, the aim is to replace physical communication wiring between battery modules and the main control unit with wireless communication, thereby reducing wire harness complexity and weight within the battery pack.

Evaluating long-term data collected from the battery using on-board processing units and cloud-based analytics infrastructure can also contribute to more detailed predictions of battery aging.