Role of BMS (Battery Management System) in Electric Vehicles

December 16, 2022

Introduction

To prevent our environment from harmful emissions, electric vehicles have come as the best substitute for a sustainable environment. Electric vehicles are propelled by electric motors, using a large rechargeable battery pack placed in the car. Electric vehicles do not produce harmful exhaust emissions, reduce noise pollution and greenhouse gases, and are a good alternative to petrol or diesel vehicles.

The performance of electric vehicles depends on various factors such as cell–voltage, battery health, etc. For achieving an efficient result from the batteries, we need a system for monitoring and controlling the various parameters related to the battery. Thus, to improve the performance of batteries a Battery Management System (BMS) is employed in every electric vehicle.

Battery–State Parameters

For the management of the batteries during an EV operation, it is necessary to monitor various states inside the battery for increasing performance and ensuring safety. The states or paraments include:

State of Charge (SoC)

As different vehicles have a fuel indicator, electric vehicles have a battery state of charge (SoC) indicator. The BMS helps in indicating and showing the actual state of charge in the battery. The SoC of a cell is a percentage value that represents the remaining charge of a battery.

State of Health Definition (SoH)

The life of battery cells deteriorates with time and the internal resistance of the battery increases while the capacity of the cell decreases. This leads to significant changes in cell performance which may make a cell unsuitable for EV applications. Thus, it is important to track the cell performance degradation, using the state of health (SoH) parameter. The battery SoH represents the slow-changing parameters, such as capacity decrease and resistance increase.

State of function (SoF)

The State of function (SoF) represents how a battery’s performance meets the application’s demands during operation. The SoF can be a percentage value or a concrete value in kW which represents whether the battery is able to fulfill the demand of the application or not. In simple words, it represents the difference between available power to demanded power.

Charge acceptance (CA)

The Charge acceptance (CA) represents the maximum charging current that the battery can accept at given conditions (SoC, SoH, temperature) and for a given charging voltage. It is very important for regenerative braking.

Key roles of a Battery Management System (BMS)

Battery Management System needs to be more advanced with the increasing performance demands of electric vehicles. Presently, Lithium-ion batteries are the heart of electric vehicles. However, Lithium-ion batteries have some issues such as overheating, and thermal imbalance which can lead to complete damage to the battery pack. The Battery Management System performs the following roles to monitor and control the battery operation and ensure safety:

Cell Monitoring:

Depending on the state of charge, each cell in a battery module experiences a different temperature which can affect the performance of the battery pack. Different sensors are employed for the direct measurement of the current, voltage and temperature parameters of an individual cell. The battery cells are always monitored when charging or discharging and any critical situation is identified and reported. The readings of different parameters measured by the sensors are used to evaluate parameters such as

State Of Charge (SoC)
State Of Health (SoH)
Energy Calculation
Current Threshold
Energy Delivered
Energy & Power Consumption
Diagnostics

Power Optimization:

The outcome of cell monitoring is the optimization of battery power. As the cell monitoring function determines SOC and SOH, the BMS of an electric vehicle keeps the SOC and SOH parameters within the specified range. When the battery is charging, the BMS determines how much current is allowed in the individual cells. When the battery is discharging, the BMS makes sure that the voltage level does not drop below a threshold.

Safety/ Protection:

The primary job of a BMS is to ensure the safe operation of a battery pack. If a thermal runway is undetected, it can cause a major mishap. As mentioned above, the BMS measures different battery parameters to optimize the power. The same data is also used to ensure the safety of the battery pack. Safe Operating Area (SOA) is very important as it is defined as the voltage, current and temperature conditions over which the battery can operate without any damage. The BMS ensures the safety of the battery pack by determining overheating and thermal management of the battery pack.

Charge/ Discharge Control:

The life of battery cells deteriorates with time. For e.g., a battery cell may get damaged by heat and starts to get charged at a lower voltage in comparison to other cells. The Battery management system determines such kinds of faults and optimizes the charging process. The BMS also performs a Cell balancing function for improving the battery life by reducing the number of charge-discharge cycles.

Communication

All the above mentions roles of Battery Management Systems such as cell monitoring, collecting information about cell parameters etc. are performed smoothly because of the communication between different parts. The type of communication depends on the application requirements. In EVs, the information about the battery and vehicle performance and different parameters are communicated by the BMS to the outside unit or driver so that timely actions can be taken in case of any critical situation.

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