The emergence of electric vehicles is driven by global warming, environmental pollution, and the energy crisis. In 2015, global electric vehicle production and shipments both exceeded 500,000 units, with my country accounting for over 370,000 of those.
Electric vehicles must have energy storage devices, and currently lithium-ion batteries are the preferred and mainstream power lithium batteries.
When lithium-ion batteries are used in series, they are susceptible to problems such as overcharging, over-discharging, overcurrent, and excessively high or low temperatures, which can cause rapid damage to the lithium-ion batteries. Therefore, a battery management system is needed to manage them.
1. Lithium-ion batteries
A lithium-ion battery is a battery made of four main materials: a positive electrode, a negative electrode, a separator, and an electrolyte, along with a casing. The positive and negative electrode materials must be capable of reversibly inserting and deintercalating lithium ions, the separator must be a lithium-ion conductive material while being electronically insulating, and the electrolyte must be a lithium-ion solution.
Typically, in positive electrode materials, a transition element undergoes a redox reaction, while in lithium metal and carbon negative electrodes, it is lithium metal that undergoes a redox reaction. During charging and discharging, lithium ions transfer back and forth between the positive and negative electrodes inside the battery, and the battery moves within the external circuit. This lithium ion transfer process is figuratively called a rocking chair, and lithium-ion batteries are called rocking chair batteries.
Figure 1 illustrates the working process of a lithium-ion battery.
Lithium-ion battery cathode materials typically use lithium-intercalated transition metal oxides, such as lithium-intercalated oxides of Ni, Co, and Mn. Anode materials, on the other hand, should be lithium-intercalated compounds with a potential as close as possible to that of metallic lithium, such as various carbon materials, SnO, SnO2, and silicon alloys.
The electrolyte commonly used is a solution of LipF6, with organic solutes such as ethylene carbonate (EC), propylene carbonate (pC), and low-density diethylene carbonate (DEC). The diaphragm is mainly made of porous composite membranes of olefin polymers. The shell material includes steel, aluminum, plastic, and aluminum-plastic film.
The typical structure of a lithium-ion battery is shown in the figure below:
Figure 2 shows a typical structure of a square battery.
Typical parameters of lithium-ion batteries include: capacity, internal resistance, and voltage; characteristic parameters of lithium-ion batteries include: cycle life, discharge platform, self-discharge rate, temperature performance, and storage performance.
Lithium-ion battery safety tests include: overcharge, short circuit, puncture, drop, immersion, low voltage, and vibration.
Lithium-ion batteries are relatively delicate. Their charging and discharging is a complex electrochemical process with multiple variables and nonlinearity. If the charging and discharging conditions are not met, events such as rapid decline in lifespan, performance degradation, fire, and explosion can easily occur because lithium-ion batteries are very sensitive to temperature, voltage, and current.
2. Development of Battery Management Systems
Early battery management systems included: the BADICHEQ and BADICOaCH systems designed in Germany starting in 1991, the battery management system used in the EV1 of General Motors in the United States, and the high-performance battery management system called BatOpt developed by ACpropulsion in the United States.
In China, the earliest and most significant research efforts involved universities leveraging their technological strengths to collaborate with major automobile and battery manufacturers. Tsinghua University developed a battery management system for its EV-6568 light electric bus; Tongji University and Beijing Xingheng jointly developed a lithium-ion battery management system; Chunlan Research Institute developed an HEV-BMS system; and Beijing Institute of Technology and Northern Jiaotong University, among others, developed distinctive battery management systems based on sub-projects of the National 863 Program's Major Project on Electric Vehicles. With the launch of the electric vehicle market, many commercially available products have achieved widespread application.
3. Research Content of Battery Management System
First, the battery management system needs to be studied, typically using a microcontroller as the core and an in-vehicle network as the distributed system. Next, sensors need to be studied, as they are used to monitor battery parameters, generally voltage, current, and temperature. Data and control transmission requires a network, typically a CAN network. Actuators are implemented through displays, relays, fans, pumps, motors, etc.
Figure 3. Hardware system schematic diagram of the battery management system
Having established a management implementation system, we now need to manage the operational system. Battery management involves three processes: discharging, charging, and resting. Resting involves temperature and safety management. Charging involves configuring charging parameters, monitoring the charging process, and protecting against temperature, voltage, and current fluctuations during charging. Discharging involves managing output power, power consumption planning, and controlling voltage, current, and temperature during use.
The same parameter is used to refer to charging, discharging, and resting: the remaining usable capacity, also known as the state of charge (SOC). The discharge process of a lithium-ion battery is a very complex electrochemical process, influenced by many factors, making the estimation of the remaining capacity extremely difficult. The difficulties mainly stem from the following aspects:
First, the battery capacity is not fixed. Under the same experience and state parameters, the battery capacity is not fixed. Second, the battery aging cannot be determined. The aging of the battery cannot be accurately calibrated at any time, and the dispersion within the battery pack cannot be accurately calibrated at any time. Third, there is randomness in the usage process. Reference [1] introduces various methods for estimating SOC.
Even if a single lithium-ion battery cell performs exceptionally well, inconsistencies still exist between individual cells during use, causing changes in the battery pack's characteristics. Currently, there is no effective solution to the dispersion phenomenon between individual cells during battery pack use. Therefore, external solutions are needed to address the balance issue of individual lithium-ion cells within the battery pack.
Currently, common balancing methods include energy consumption balancing, charging balancing, and energy transfer balancing. The most typical and widely used is energy consumption balancing, which utilizes a heating resistor for bypass shunting, as shown in the diagram below:
Figure 4. Schematic diagram of the principle of energy consumption balancing
Charging equalization involves using a small charger to fully charge each individual battery cell at the end of the charging process. Due to the difficulty of measuring State of Charge (SOC), despite significant research and development, energy transfer equalization has not yet reached practical applications.
Of course, this alone is not enough for a battery management system. Batteries will heat up during use, and excessively high temperatures will render lithium-ion batteries unusable—a desirable outcome. Therefore, the initial battery management system also incorporated thermal management. Later, it was discovered that charging and discharging could not continue when the battery temperature dropped too low in low-temperature environments, leading to the implementation of heating management.
As the application of batteries expands, battery safety issues increase, leading to the need for safety management. Initially, safety management involved monitoring; the Battery Management System (BMS) sent battery data to a monitoring center, which then used this data to identify potential safety hazards. This has evolved to include the BMS itself providing early warnings about safety risks.
During the use of batteries, maintenance, replacement of individual cells, and equalization are always required. These tasks require diagnosis. If the BMS has already done the diagnosis and prepared the data beforehand, the corresponding work will become much simpler. Therefore, the battery management system has added the functions of fault diagnosis and reporting [2].
With the increase in retired batteries, problems have emerged in the secondary use and recycling of batteries [3]. A lot of research needs to be done on the grouping of secondary batteries, and the BMS has taken on the management function of grouping optimization.
The progress of battery research and development also depends on the problems and phenomena found during battery use and the selection based on actual use. Therefore, the battery management system has added the function of battery technology selection[4].
4. Future Prospects for Battery Management Systems
Measurement is fundamental to battery management, and increasingly precise, high-resolution technologies are being applied to battery management systems. Research on State of Charge (SOC) estimation has also evolved from solely relying on ampere-hour integration to include other methods such as joule integration. With the increasing number of battery management functions, the rise of multi-level battery management systems is noteworthy.
The architecture has evolved from a master-slave structure to one where each independent replacement unit can possess complete battery management system functionality. Beyond the battery system itself, vehicle battery management and back-end server-based battery management programs are also emerging.
Furthermore, it's worth noting that battery management systems are no longer passively protecting batteries, but rather optimizing usage and the operating environment. Temperature management optimizes the operating environment, while parameter analysis optimizes usage. With the industry's development, we can expect to see more and better battery management technologies and products emerge.
