Management systems to improve electric vehicle battery performance

1 Design of Electric Vehicle Battery Management System

With the increasing shortage of energy, rising oil prices, and urban environmental pollution, the development and utilization of new energy to replace oil has been increasingly valued by governments around the world. In the new energy system, the battery system is an indispensable and important component. In recent years, electric bicycles, hybrid vehicles, electric vehicles, fuel cell vehicles, etc. powered by lithium batteries have received increasing attention from the market. The application of power batteries in the field of transportation is of great significance for reducing greenhouse gas emissions, reducing air pollution, and the application of new energy. Among them, lithium batteries have attracted more and more attention due to their high energy density, high number of repeated cycles, light weight, and green environmental protection. Therefore, they have been widely used in portable handheld devices such as mobile phones, laptops, and power tools, and have begun to enter high-power applications such as electric vehicles and electric vehicles, becoming a hot spot for the development of electric vehicles around the world.

However, because lithium batteries may be damaged or even catch fire or explode under abuse conditions such as heating, overcharge/over-discharge current, vibration, and extrusion, safety issues have become the main constraint on the commercialization of power lithium batteries. Safety standards, safety evaluation methods, safety and reliability control of battery manufacturing processes, and improving battery safety and reliability through the optimization of positive and negative electrode materials, electrolytes, and separators for safe, low-cost, and long-life lithium-ion batteries are the key to ensuring the safety, reliability, and practicality of large-scale power lithium-ion batteries. As the core component of battery protection and management, the battery management system must not only ensure the safe and reliable use of the battery, but also give full play to the battery's capabilities and extend its service life. As a bridge between the battery and the vehicle management system and the driver, the battery management system plays an increasingly critical role in the performance of electric vehicles.

2 Main functions of battery management system

The battery management system is closely integrated with the power battery of the electric vehicle, and it constantly detects the voltage, current, and temperature of the battery. It also performs leakage detection, thermal management, battery balancing management, alarm reminders, calculates the remaining capacity and discharge power, and reports the SOC & SOH status. It also uses algorithms to control the maximum output power according to the voltage, current, and temperature of the battery to obtain the maximum mileage, and uses algorithms to control the charger to charge with the optimal current. It communicates in real time with the on-board master controller, motor controller, energy control system, on-board display system, etc. through the CAN bus interface. Figure 1 is a simple block diagram of the battery management system.

Figure 1 Simple block diagram of a battery management system

Basic functions of battery management system:

1) Monitor the working conditions of single cells, such as single cell voltage, working current, ambient temperature, etc.

2) Protect the battery to prevent it from shortening its life, becoming damaged, or even exploding, catching fire, or other accidents that endanger personal safety when the battery is operated under extreme conditions.

Generally speaking, the battery management system must have the following circuit protection functions: overvoltage and undervoltage protection, overcurrent and short circuit protection, over-temperature and over-temperature protection, and multiple protections for the battery to improve the reliability of the protection and management system (hardware-implemented protection has high reliability, software-implemented protection has higher flexibility, and the protection of key components of the management system fails provides users with a third level of protection). These functions can meet the needs of most mobile phone batteries, power tools, and electric bicycle applications.

3 Electric vehicles pose higher challenges to battery management systems

The electric vehicle battery integration system is an open power system that communicates through the automotive-grade CAN bus and works with the vehicle management system, charger, and motor controller to meet the people-oriented safe driving concept of the car. Therefore, the automotive-grade battery management system must meet the requirements of TS16949 and automotive electronics, achieve high-speed data acquisition and high reliability, automotive-grade CAN bus communication, high anti-electromagnetic interference capability (the highest level of EMI/EMC requirements), and online diagnostic functions.

Its main functions are:

High-speed acquisition of battery voltage, temperature and other information; achieving high-efficiency battery balancing, giving full play to the capacity of the battery integration system to increase the life of the battery integration system, while reducing heat generation;

Estimation and display of battery health and remaining power;

Highly reliable communication protocol (automotive-grade CAN communication network);

Powertrain technology must ensure that the battery is used safely, give full play to its potential, ensure battery performance, and extend battery life;

Battery temperature and heat dissipation management means that the battery system works in an environment with relatively stable temperature;

Leakage detection and complex ground wire design.

Since the distribution environment of batteries in electric vehicles is very complex and they are in a high-voltage and high-power working state, the EMI/EMC requirements are very high, which brings greater challenges to the design of battery management systems.

4 Hierarchical and modular design of electric vehicle battery systems

Since the battery system of electric vehicles is integrated with hundreds or thousands of battery cells, considering the space, weight distribution and safety requirements of the car, these battery cells are divided into standard battery modules, distributed in different positions of the car chassis, and managed by the powertrain and central processing unit; each standard battery module is also composed of multiple cells connected in parallel and series, managed by the module's electronic control unit, and the information of the battery module is reported to the central processor and powertrain unit through the CAN bus. After the central processor and powertrain unit process this information, they report the final information about the integrated system, such as the remaining power, health status, and battery capacity related information, to the vehicle management system through the CAN bus. The hierarchical and modular design of the battery system of electric vehicles requires the hierarchical and modular design of the battery management system (Figure 2).

Figure 2 Hierarchical and modular design of electric vehicle battery management system

5. Chip Integration Technology for Battery Management System

The reliability requirements of automotive battery systems are extremely high, especially for the high-voltage monitoring and battery balancing parts. Due to the lack of integrated solutions, many solutions are made of discrete components, resulting in: poor component matching and reduced signal acquisition accuracy; an increase in external nodes, making automated testing difficult, increasing testing costs, reducing test coverage, and low system reliability; the power consumption of external components is difficult to control; and the system is large in size and high in cost.

O2Micro provides the world's first single-chip protection and detection solution OZ89xx that supports more than 5 battery cells in series. The solution also supports multi-chip cascade applications. Currently, the battery management system solution using this chip has been successfully used in the battery module electronic control unit of pure electric vehicles and hybrid vehicles.

Table 1 introduces the comparison between discrete and integrated solutions using a standard battery module as an example.

It can be seen that integrated chip solutions play a very important role in improving system reliability and reducing costs, and are the core of hardware design technology in battery integration technology.

6 Conclusion

In the future, power lithium batteries will have broad prospects in the field of electric vehicles, and the battery management system will play a key role in bridging the safe use of batteries and communication with vehicle management. Battery management technology includes hardware design technology and software design technology, among which high-voltage mixed signal processing technology and chip design are the core of hardware design, which is not only the key to ensure high reliability, high speed, and high precision signal acquisition and processing in the automotive environment, but also the key to improving test coverage, supporting online detection and reducing costs; the core of the software includes battery management algorithms, communication protocol support, and powertrain-related technologies. O2Micro is one of the world's major battery management solution providers. With its many years of experience in chip design and solution design in battery protection and management, it has mastered internationally advanced battery management technology, provided high-quality technical services to global battery manufacturers and system manufacturers, and contributed to the development of China's electric vehicles.