Analysis of Electric Vehicle Lithium Battery BMS System

The emergence of electric vehicles is driven by global warming, environmental pollution and energy crisis. In 2015, the global production and shipment of electric vehicles exceeded 500,000 units, of which China exceeded 370,000 units. Electric vehicles must have energy storage devices. Currently, lithium-ion batteries are the first choice and mainstream of power batteries. When lithium-ion batteries are used in series, there are problems such as overcharge, over-discharge, over-current, and excessively high or low temperatures, which will cause rapid damage to lithium-ion batteries, so a battery management system is needed to manage them.

1. Lithium-ion battery

Lithium-ion batteries refer to batteries made of four main materials: positive electrode, negative electrode, separator, electrolyte and shell. The positive and negative electrode materials must be able to reversibly embed and de-embed lithium ions, the separator must be lithium ion conductive and electronically insulating, and the electrolyte must be a lithium ion solution.

Usually, the positive electrode material contains a transition element that undergoes redox reaction, while the metal lithium and carbon negative electrode contain metal lithium that undergoes redox reaction. During the charge and discharge process, lithium ions are transferred back and forth between the positive and negative electrodes inside the battery, and the battery moves in the external circuit. Some people figuratively call this lithium ion transfer process a rocking chair, and lithium-ion batteries are called rocking chair batteries.

Figure 1 Schematic diagram of the working process of lithium-ion batteries

The positive electrode material of lithium-ion batteries generally uses lithium-intercalated transition metal oxides, such as lithium-intercalated oxides of Ni, Co, and Mn. The negative electrode material should be a lithium-intercalated compound with a potential as close to that of metallic lithium as possible, such as various carbon materials, SnO, SnO2, silicon alloys, etc.

The electrolyte generally uses a solution of LiPF6, and the solute is an organic matter, commonly used are ethylene carbonate (EC), propylene carbonate (PC) and low-density diethylene carbonate (DEC), etc.; the diaphragm is mainly made of olefin polymers to make a porous composite film; the shell material is steel, aluminum, plastic, aluminum-plastic film, etc. The typical structure of a lithium-ion battery is as follows:

Figure 2 Typical structure of a square battery

Typical parameters of lithium-ion batteries include: capacity, internal resistance, voltage; characteristic parameters of lithium-ion batteries include: cycle life, discharge platform, self-discharge rate, temperature performance, storage performance, etc. Safety tests for lithium-ion batteries include: overcharge, short circuit, puncture, drop, immersion, low voltage, vibration, etc.

Lithium-ion batteries are relatively delicate, and their charging and discharging is a multivariable, nonlinear and complex electrochemical process. If the charging and discharging conditions are not met, they are prone to rapid reduction in life, performance degradation, fire, explosion and other incidents, because lithium-ion batteries are very sensitive to temperature, voltage, current, etc.

2. Development of battery management system

Early battery management systems include: the BADICHEQ and BADICOaCH systems designed in Germany in 1991, the battery management system used by General Motors EV1 in the United States, and the high-performance battery management system called BatOPt developed by AC Propulsion in the United States.

At first, some universities in China relied on their own technological advantages to conduct some research in cooperation with some large automobile and battery manufacturers. Tsinghua University equipped the EV-6568 light electric bus with a battery management system, Tongji University and Beijing Xingheng cooperated to develop a lithium-ion battery management system, Chunlan Research Institute developed a HEV-BMS system, and Beijing Institute of Technology and Northern Jiaotong University, relying on the major special projects of the National 863 Plan for electric vehicles, also developed a unique battery management system. With the launch of the electric vehicle market, many commercial products have been widely used.

3. Research content of battery management system

First, we need to study the battery management system, which is generally based on the microcontroller as the core and the vehicle network as the distributed system. Then we need to study the sensor, because we need to detect the parameters of the battery. Generally, we detect voltage, current, and temperature. The transmission of data and control requires a network, generally using a CAN network. The actuator is realized through display screens, relays, fans, pumps, motors, etc.

Figure 3 Schematic diagram of the hardware system of the battery management system

With the management implementation system, a management operation system is needed. Battery management is divided into three processes: discharge, charge and rest. Rest involves temperature and safety management. Charging involves the configuration of charging parameters, monitoring of the charging process, and protection of temperature, voltage and current during the charging process. The discharge process involves the management of output power, the management of power planning, and the management of voltage, current and temperature during use.

The same parameter is required for charging, discharging and resting, which is the remaining available power, also known as the state of charge (SOC). The discharge process of lithium-ion batteries is a very complex electrochemical process, which is affected by many factors. It is very difficult to estimate the remaining power. The difficulties mainly come from the following aspects:

First, the capacity of the battery is not fixed. Under the same experience and state parameters, the capacity of the battery 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 degree in the battery pack cannot be accurately calibrated at any time; third, the randomness of the use process. The literature introduces various SOC estimation methods.

During the use of lithium-ion battery packs, even if the performance of a single battery cell is superior, there are inconsistencies between cells, and the characteristics of the battery pack will change during use. Currently, there is no effective solution to the phenomenon of dispersion between cells during the use of the battery pack, so an external solution is needed to solve the problem of balancing each single lithium battery in the battery pack.

At present, the common equalization methods include energy consumption equalization, charging equalization and energy transfer equalization. The most typical and widely used is energy consumption equalization, which uses heating resistor bypass shunting, the principle is as follows:

Figure 4 Schematic diagram of energy consumption balance

Charge equalization is to use a small charger to fully charge each single battery at the end of charging. Energy transfer equalization is difficult to measure SOC, and although there are many research and development, it has not yet entered into practical products.

Of course, it is not enough for the battery management system to do this. The temperature of the battery will rise during use. If the temperature is too high, the lithium-ion battery can no longer be used, which is not desirable. Therefore, the initial battery management system added the function of heat dissipation management. Later, it was found that in a low temperature environment, the battery temperature was too low and charging and discharging could not continue, so heating management was implemented.

As the scope of battery use expands, battery safety issues increase, and so there is the issue of safety management. Initially, safety management was monitoring, where the BMS sent battery data to the monitoring center, which then judged safety hazards based on the data. This has further evolved to early warnings of safety issues for the BMS itself.

During use, batteries always need maintenance, cell replacement, and balancing. These tasks require diagnosis. If the BMS has completed the diagnosis and prepared the data before it is needed, the corresponding work will become much simpler. Therefore, the battery management system has added the function of fault diagnosis and reporting.

With the increase in retired batteries, problems have arisen in the cascade utilization and recycling of batteries. A lot of research is needed on the matching of batteries for cascade utilization, and BMS has assumed the management function of matching optimization.

The progress of battery research and development also depends on the problems and phenomena discovered during the use of the battery, and on the choices made during actual use. Therefore, the battery management system has added the function of battery technology selection.

4. Development Prospects of Battery Management System

Measurement is the basis of battery management. More and more accurate and high-resolution technologies are applied to battery management systems. Research on SOC estimation has also evolved from the single-color ampere-hour integration to other methods such as joule integration. Battery management functions are increasing, and it is worth noting that multi-level battery management systems are emerging.

From the master-slave structure, each independent replacement unit has developed to have complete battery management system functions. In addition to the battery system, vehicle battery management and backend server battery management programs are also emerging. In addition, it is worth noting that the battery management system is no longer passively protecting the battery, but optimizing the use and use environment. Temperature management is to optimize the use environment, and parameter deduction is to optimize the use. With the development of the industry, we can expect more and better battery management technologies and products to emerge.