Why is a Battery Management System needed in Electric Vehicles?

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Source of the cover image: Buccolini, Luca et al. “Battery Management System (BMS) simulation environment for electric vehicles.” 2016 IEEE 16th International Conference on Environment and Electrical Engineering (EEEIC) (2016): 1-6.

This article is a part of EVreporter Learning series. We explore the following basic questions regarding the Battery Management System (BMS) – the brain of the EV battery:

What is a Battery Management System?

What are the Primary Functions of the BMS for an EV battery?

What is a Battery Management System (BMS)?

BMS is an electronic system that manages a rechargeable battery to ensure it operates safely and efficiently. BMS is designed to monitor the parameters associated with the battery pack and its individual cells, apply the collected data to eliminate safety risks and optimise the battery performance.

As you can see in the picture, the Battery Management System is an embedded system that has a number of electronic components on a circuit board. An embedded system comprises of purpose-built electronics along with purpose-built software to enable a specific application. We will elaborate further on the functionality of BMS in the subsequent sections.

What are the Primary Functions of the BMS for an EV battery?

1. Safety

Electric vehicles run on high voltage Lithium-ion battery packs. Lithium-ion batteries have higher energy density (i.e. 100-265 Wh/kg) than other battery chemistries. These batteries come with a risk of catching fire under unusual circumstances. It is imperative to operate the EV batteries in pre-defined safe limits to ensure the safety of the user as well as the vehicle.

The Battery Management System continuously monitors parameters such as temperature, voltage and current in and out of the pack to ensure it is being operated in safe conditions the entire time. BMS is responsible for thermal management of the battery and monitors its temperature continuously. If required, BMS can adjust cooling and trigger other safety mechanisms to cease operations and minimize the risk. e.g. in Hyundai Kona Electric, if overheating of the battery pack is detected by the BMS, the vehicle’s power output is automatically limited and the car is put in fail-safe mode.

Overcharging of lithium-ion cells can also lead to thermal runaway and potentially an explosion. BMS continuously monitors the voltage of the pack as well as individual battery cells and controls the supply of the current to avoid overcharging. BMS can enforce the limits of maximum charge or discharge current according to temperature.

Sensing electrical isolation – The BMS also checks that the vehicle chassis is completely isolated from the high voltage battery pack at all times to prevent the user from getting an electric shock.

Also Read: Vaakulab’s Made in India BMS solutions

2. Performance Optimization

BMS is responsible for optimising the performance of the battery pack.

Lithium-ion batteries perform best when their State of Charge (SoC) is maintained between the minimum and maximum charge limits defined in the battery profile. Overcharging as well as deep discharging degrades the capacity of the battery, thereby shortening its life. At the time of charging, BMS determines how much current can safely go in and communicates the same to the EVSE (Electric Vehicle Supply Equipment or the Charger). During discharge of the battery, BMS would communicate with the motor controller to avoid the cell voltages reaching too low. The vehicles can show a corresponding alert to the user to charge the battery pack. The BMS also controls the recharging of the battery pack by energy generated through regenerative braking.

Individual cells in the battery pack can develop differences in capacity with time, which amplify with each charge/discharge cycle. This imbalance limits the amount of energy that can be derived from the battery, and also how much the battery pack can be charged. Cell Balancing is needed to maintain the cells at equal voltage levels and maximise the capacity utilization of the battery pack. Measurement of individual cell voltages by BMS indicates their relative balance and acts as a pointer to how much charge equalization is required. The BMS performs cell balancing by draining excess energy from cells that are more charged than others, through active or passive balancing techniques.

3. Health Monitoring and Diagnostics

The BMS uses the collected data points (temperature, voltage, current etc.) to estimate the State of Charge and State of Health (SoH) of the battery pack. The SoC refers to available energy in the battery and determines how far the vehicle can go before needing to recharge. The SoH measures the current condition of the battery as compared to its original capacity and indicates the battery’s suitability for the application. Both SoC and SoH are presented as percentages.

BMS also checks for anomalies in the parameters and behaviour of the cells and the battery pack. It stores the error codes and logs diagnostic information that helps fix any issues with the battery. The BMS can either take necessary corrective actions or trigger failsafe mechanisms to preserve the health of the pack.

4. Communication

The BMS is responsible for communicating with other ECUs (Electronic Control Units) in the vehicle. It relays the necessary data about the battery parameters to the motor controller to ensure the smooth running of the vehicle. In case of AC charging, BMS communicates with the onboard charger to monitor and control the charging of the battery pack.

For DC charging, a communication link is established directly between the EVSE and the BMS. BMS communicates the required output voltage and current levels to the EVSE, and sends instructions to start and stop the charging process.

Also Read: Guide to EV Charging and Standards in India