EV charging and battery management - ensuring optimal performance and lifespan

in English

Electric and hybrid electric vehicles rely on high-voltage battery packs to power their electric motors. These packs are crucial as they store the energy required to run the vehicle. To ensure the optimal functioning of these battery packs, a battery management system (BMS) is used. The BMS has multiple roles, including prolonging the battery's lifespan and increasing the vehicle's driving range. BMS achieves this by regulating the charging and discharging of the battery packs.

The BMS provides critical information about the battery's state of charge (SoC), state of health (SoH), and state of function (SoF) to the device's control unit or user. This information enables informed decisions to be made about the battery's usage and charging requirements. As a result, the BMS plays a vital role in assuring the safe and efficient operation of electric vehicles. By monitoring and controlling the battery's SoC, temperature, and voltage levels, the BMS maximises the battery's lifespan, enhances vehicle performance, and improves safety.

Li-ion Battery charge profile

Lithium-ion batteries are the most commonly used technology in electric vehicles due to their high energy density (100-265wh/kg) compared to other battery types. Li-ion batteries are designed to be charged up to a specific voltage and then stopped to prevent instability and potential fires. It's essential to ensure that each cell or module's voltage, current, and temperature does not exceed specific limits when charging or discharging a battery. These limits are known as SOA limits, and BMS ensures that residual energy in a battery is optimally utilised. BMS also protects batteries from deep discharge and over-voltage to prevent dangerous thermal runaway conditions and damage to the battery pack.

Li-ion batteries have different states of charging based upon their voltage, and Constant-current constant-voltage (CCCV) charging is a popular method accomplished by BMS. This efficient and safe charging method limits the charging current to a safe level and prevents overcharging. As shown in figure 1, CCCV charging involves a constant current charging stage and a constant voltage charging stage. During the constant current stage, a constant current is applied to the battery, gradually increasing the battery voltage as it charges. Once the battery reaches a specific voltage, the charging switches to the constant voltage stage. The charging voltage is held constant, and the charging current gradually decreases until the battery is fully charged.

Figure 1: Constant-current constant-voltage battery charging scheme

Functional block diagram of BMS

Figure 2 illustrates the BMS block diagram, which shows how the different components of the system work together to enhance the battery pack's performance and safety. The BMS comprises a Microcontroller/Microprocessor that processes battery information and regulates the charging and discharging procedures. The system also includes sensors and communication interfaces that gather and transmit data to other devices or systems.

The Cell Monitoring Units measure individual battery cells' voltage, current, and temperature and send this information to the BMS controller for processing. The controller then uses this data to manage the charging and discharging of the battery pack. It can also provide safety features like overcharge protection, over-discharge protection, and short circuit protection to prevent damage to the device or battery. Furthermore, the communication interface permits the BMS to exchange data with other systems or devices, such as the vehicle's main controller or a remote monitoring system.

Figure 2: Functional block diagram of BMS

Primary function of EV-BMS

A BMS for an Electric Vehicle battery manages and controls the battery's charging and discharging, monitors its state of charge and health, and ensures safe operation. The BMS's safety features are critical for EV safety, preventing dangerous situations. The BMS monitors the battery pack constantly, taking action to prevent damage or danger, making EVs safe and reliable transportation.

The BMS has several safety features:

Cell balancing: The BMS ensures that each cell in the battery is charged and discharged equally, to prevent overcharging or undercharging of any cell, which could lead to thermal runaway or fire. This helps to maintain the overall health and longevity of the battery.
State of Charge (SOC) estimation: The BMS constantly monitors the SOC of the battery, which is the amount of energy stored in the battery at any given time. This information is used to provide accurate range estimates to the driver and to prevent over-discharging of the battery.
Thermal management: The BMS monitors the temperature of the battery and manages the cooling and heating systems to ensure that the battery is operating within safe temperature limits.
Safety monitoring: The BMS continuously monitors the battery for any faults or malfunctions, and can take action to prevent damage to the battery or to the vehicle. If a fault is detected, the BMS can shut down the battery pack to prevent further damage or danger.
Overvoltage and undervoltage protection: The BMS constantly monitors the voltage of each cell in the battery pack. If any cell voltage exceeds safe limits, the BMS will take action to prevent overcharging, which could lead to thermal runaway or fire. Similarly, if any cell voltage drops below safe limits, the BMS will take action to prevent over-discharging, which could lead to reduced battery life or damage.
Overcurrent protection: The BMS monitors the current flowing in and out of the battery pack. If the current exceeds safe limits, the BMS will take action to prevent damage to the battery or to the vehicle.
Thermal management: The BMS monitors the temperature of the battery pack and can activate cooling or heating systems to maintain safe temperatures. If the temperature exceeds safe limits, the BMS will take action to prevent damage to the battery or to the vehicle.
Communication with the vehicle: The BMS communicates with other systems in the vehicle, such as the motor controller and the charging system, to ensure that they are operating safely and within their limits.

Battery management and EV charging types

Battery management systems come in different levels of complexity and can incorporate various technologies. The type of BMS used in an electric vehicle depends upon several factors, such as the battery pack's size, desired performance, safety levels, and budget for the system.

There are two types of EV charging: AC (alternating current) and DC (direct current) charging. With AC charging, the charging cable connects to an AC power source, such as a standard wall outlet or an AC charging station. The current flows through the cable and into the onboard charger, which converts it from AC to DC and sends it to the battery via the BMS.

The BMS manages and protects the battery during charging and discharging, ensuring it lasts as long as possible. Components that monitor the battery include devices that measure voltage and current, isolate signals, and ensure safety. With the prevalence of lithium-ion batteries in electric and hybrid electric vehicles, it's crucial to have a system that protects and monitors the battery.

Off-board and on-board BMS

There are two types of BMS used for EV charging: an off-board and on-board. The main difference between an off-board BMS and an on-board BMS for an EV is the location of the BMS relative to the battery and the charging process. An off-board BMS is typically located in a charging station, separate from the vehicle, and manages the charging process of the battery whilst it is not connected to the vehicle. The BMS in an off-board charger may also be responsible for managing multiple charging stations and communicating with a central management system to optimise the charging process.

On the other hand, an on-board BMS is located within the EV and manages the charging process of the battery whilst it is connected to a charging station. The battery can be charged using a standard power outlet with the help of an onboard charger unit. To charge the battery packs, an on-board AC/DC converter module is used. This module converts the alternating current (AC) from an external power source, such as a charging station or wall outlet, into direct current (DC) that is used to charge the battery packs.

The cooperative operation of the BMS and the AC/DC converter module is crucial for the reliable and efficient charging of the battery packs in electric or hybrid electric vehicles. The BMS in an on-board charger may also be responsible for managing the battery's state of charge, optimising energy usage, and monitoring the battery's health and performance.

For the majority of users, a power supply of 120 VAC at 15 to 20 A is easily accessible and all onboard chargers should be capable of handling it. However, as charging time is a crucial factor for car drivers, some users can opt for 240 VAC which can provide faster charging times, but it will necessitate a more powerful power source.

Both off-board and on-board BMS have their advantages and disadvantages. Off-board BMS can be more cost-effective as they can manage multiple charging stations and don't require the complexity of on-board BMS. On the other hand, on-board BMS provides more control over the charging process and allows for more precise management of the battery's state of charge. Ultimately, the goal of both types of BMS is to ensure safe and efficient charging, extending the battery's life and improving overall performance.