A Battery Management System (BMS) is an electronic control system that monitors, controls, and optimizes the performance, safety, and lifespan of an electric vehicle’s battery pack. It regulates the heat generated by the battery and enhances the battery's performance. The battery pack is one of the important components of an electric vehicle and constitutes almost 40% of the total vehicle cost. The BMS comprises of control unit that monitors the battery pack. Each cell is monitored for its current, voltage, and temperature and is processed using a control algorithm. The output is in the form of state of charge (SOC), state of health (SOH), thermal management, and power optimization. All this information is communicated with other electronic control units (ECU) and is also displayed in the display unit.

Battery Management System for Electric Vehicles

Lithium-ion batteries are usually preferred for use in electric vehicles because of their high power density, low self-discharge, and relatively low cost. However, individual battery cells alone cannot provide enough power to operate an electric vehicle. Therefore, many cells are combined in series and parallel to create a module. Many modules are connected to form a battery pack. At the pack level, it becomes difficult to manage the performance of lithium-ion battery packs as the cells can get charged and discharged at different rates. Also, the cells could be operating under different conditions, due to their different operational state in terms of temperature, state of charge (SoC), and state of health (SoH). Hence a battery management system is required to monitor the charge rate across the whole pack to cell level.

Types of BMS in Electric Vehicles

Two types of BMS architecture can be employed in electric vehicles -

• Centralized battery management system: A centralized BMS architecture uses a single BMS unit for monitoring and managing the entire battery pack. In this configuration, sensors are distributed throughout the battery pack to measure parameters such as voltage, current, and temperature of individual cells or modules. The sensors feed the data to the central control unit which is then processed and the decision is made on the battery management.
• Distributed battery management system: In a distributed BMS architecture, each battery cell or module has its own BMS unit responsible for monitoring and managing its parameters such as voltage, current, and temperature. These individual BMS units communicate with each other to coordinate the battery management functions across the entire battery pack.

Functions of Battery Management System

A BMS has several functions that include

Cell Balancing: Individual cells of a battery pack may have slight differences in capacity, internal resistance, and aging characteristics, which may lead to cell voltage imbalances over time. Cell balancing is the process of equalizing the state of charge (SoC) or voltage across all cells within the battery pack to maximize the pack's usable capacity, extend its lifespan, and ensure safe operation.

There are two methods of cell balancing:

• Passive Balancing: Passive balancing is the process of dissipating excess energy from cells with higher voltage levels. This is achieved with the help of passive components such as resistors or bypass diodes connected across individual cells. When an individual cell's voltage exceeds a predetermined threshold value, the passive balancing circuit allows current to flow through the resistor or diode, bypassing the excess charge and dissipating it as heat. This way of cell balancing is relatively simple and cost-effective but offers lower efficiency when dealing with large voltage differentials.
• Active Balancing: Active balancing is a means of redistributing energy between cells to equalize their voltages. This is done by transferring charge between cells using active components such as DC-DC converters or charge pumps. The voltage of each cell is monitored and whenever necessary, energy is transferred from cells with higher voltage to those with lower voltage. This method of cell balancing is more efficient than passive balancing and can handle larger voltage differentials, making it suitable for high-capacity battery packs.

Thermal Management: Each lithium-ion battery cell within the battery pack has an acceptable voltage range. A cell operating out of this temperature range will cause performance degradation and irreversible damage to the cells. In extreme cases, it can cause thermal runaway or even explosion. A BMS controls the temperature of the battery through heating and cooling. Temperature sensors are installed in the pack to provide real-time cell temperature information. The BMS uses this information to distribute coolant where it is needed to maintain the ideal temperature range. Common coolants used include air, water or glycol, dielectric oil, and refrigerant. During low-temperature operation, heating systems are employed to maintain the battery cells within an optimal temperature range. These systems include electric heaters or heat exchangers that warm the battery pack to improve performance and prevent damage from low temperatures. Battery packs include thermal insulation materials to minimize heat transfer between the battery pack and the surrounding environment. The BMS utilizes control algorithms to manage thermal conditions within the battery pack. These algorithms adjust charging and discharging rates, activate cooling or heating systems as needed, and implement strategies to prevent overheating or overcooling of the cells.

Advantages of BMS

• Extended Battery Life: A BMS monitors various battery parameters such as current, voltage, and temperature thereby ensuring its operation within the safe limits. This prevents overcharging, over-discharging, and conditions that could degrade the battery's health, thus extending the lifespan of the battery.
• Safety: The commonly used lithium-ion batteries in EVs are prone to thermal runaway if not managed properly. This can lead to fire or explosion. The BMS continuously checks the temperature of individual cells and implements measures to prevent overheating or overcharging, thereby enhancing the safety of the vehicle.
• Performance Optimization: The BMS optimizes the performance of the battery pack by ensuring that each cell operates at its optimal voltage and temperature range. It also distributes the load evenly among cells, maximizing the overall efficiency and performance of the battery pack.
• Range Estimation: The BMS monitors the energy consumption and estimates the state of charge (SoC). This information provides the driver with accurate range estimations. This is essential for planning trips and avoiding situations where the vehicle runs out of charge unexpectedly.
• Fault Diagnosis and Maintenance: The BMS can detect faulty cells or modules within the battery pack and alert the driver for maintenance. This early detection helps in preventing further damage to the battery pack, allowing timely maintenance or replacement, and reducing downtime and repair costs.

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