Analysis of electric vehicle lithium battery BMS system

The emergence of electric vehicles is driven by global warming, environmental pollution and energy crises. In 2015, global electric vehicle production and shipments exceeded 500,000 units, of which China exceeded 370,000 units. Electric vehicles must have electrical energy storage devices. Currently, lithium-ion batteries are the first choice and mainstream for power batteries. Lithium-ion batteries have problems such as overcharging, over-discharging, over-current, and over-temperature when used in series, which can cause rapid damage of lithium-ion batteries, so battery management systems are required for management.

A lithium ion battery refers to a battery made of four main materials and a casing of a positive electrode, a negative electrode, a separator, and an electrolyte. The positive and negative materials must be capable of reversibly intercalating and deintercalating lithium ions. The separator must be lithium ion conductive and electronically insulated. The electrolyte must be a lithium ion solution.

Usually, a transition element in the positive electrode material undergoes a redox reaction, and a metal lithium and a carbon negative electrode are redox reactions of metallic lithium. During the charging and discharging process, lithium ions are transferred back and forth between the positive and negative electrodes inside the battery, and the battery moves in an external circuit. Some people have vividly turned this lithium ion transfer process into a rocking chair, and a lithium ion battery as a rocking chair type battery.

Figure 1 Diagram of the working process of lithium ion battery

Lithium-ion battery cathode materials generally use lithium intercalation transition metal oxides, such as Ni, Co, Mn lithium intercalation oxide. For the negative electrode material, a lithium intercalation compound having a potential as close as possible to metallic lithium, such as various carbon materials, SnO, SnO2, silicon alloy, or the like, should be selected.

The solution of LiPF6 is commonly used in the electrolyte. The solute is organic. Commonly used are ethylene carbonate (EC), propylene carbonate (PC) and low density diethylene carbonate (DEC). The separator is mainly made of olefin polymer. Porous composite membrane; the outer casing material is steel, aluminum, plastic, aluminum plastic film and the like. 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 are: capacity, internal resistance, voltage; lithium ion battery characteristic parameters are: cycle life, discharge platform, self-discharge rate, temperature performance, storage performance. Lithium-ion battery safety tests include: overcharge, short circuit, acupuncture, drop, water immersion, low pressure, vibration, etc.

Lithium-ion battery is more delicate, its charge and discharge is a multi-variable, nonlinear and complex electrochemical process. If it can not meet the requirements of its charge and discharge conditions, it is easy to have rapid life decline, performance degradation, fire, explosion and other events, because lithium Ion batteries are sensitive to temperature, voltage, current, and the like.

The early battery management systems included the BADICHEQ and BADICOaCH systems that were designed in Germany in 1991, the battery management system used by General Motors EV1, and the high-performance battery management system called BatOPt developed by AC Propulsion.

The earliest domestic universities mainly rely on their own scientific and technological advantages to unite some large auto and battery manufacturers to carry out some research work. Tsinghua University has developed a battery management system for EV-6568 light electric buses, and Tongji University and Beijing Xingheng have jointly developed. The lithium-ion battery management system and the Chunlan Research Institute have developed the HEV-BMS system, Beijing Institute of Technology and Northern Jiaotong University, etc., relying on the National 863 Program's major sub-projects for electric vehicles, and also developed a unique battery management system. With the launch of the electric vehicle market, many commercial products have been used in large quantities.

First of all, to study the battery management system, generally research on the microcontroller as the core, the vehicle network is the distribution system. Then study the sensing because the parameters of the battery need to be detected. Generally, voltage, current, and temperature are detected. The transmission of data and control needs to be implemented by the network, generally using a CAN network. The actuator is realized by a display screen, a relay, a fan, a pump, a motor, and the like.

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

With a managed implementation system, a running system that needs to be managed. The management of the battery is divided into three processes of discharging, charging and standing. Resting involves temperature and safety management. Charging involves the configuration of charging parameters, the monitoring of the charging process, the protection of the temperature, voltage and current during the charging process. The discharge process involves the management of output power, the management of electricity planning, and the management of process voltage, current, and temperature.

The charging and discharging will need to refer to the same parameter, that is, the remaining available power, also called the state of charge (SOC). The discharge process of lithium-ion batteries is a very complicated electrochemical process, which is affected by many factors. The estimation of remaining power is very difficult. 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 within the battery pack cannot be accurately determined. Calibration at any time; third is the randomness of the use process. The literature describes various methods for estimating SOC.

During the use of lithium-ion battery packs, even if the performance of a single-cell battery is superior, there will be inconsistencies between the monomers. The battery pack will also change its characteristics during use. Currently, the battery pack is in use during use. There is no dispersive phenomenon between them, and there is no effective solution. Therefore, it is necessary to solve the problem of balance of each single-cell lithium battery in the battery pack.

At present, the general equalization methods have energy consumption balance, charge balance and energy transfer balance. The most typical and widely used is energy consumption equalization. This method uses the heating resistor to bypass the shunt. The principle is as follows:

Figure 4 Schematic diagram of energy balance

Charging equalization is the use of a small charger for each single cell to fully charge it at the end of charging. Energy transfer equalization Due to the difficulty of SOC measurement, although there is a lot of research and development, it has not yet entered a practical product.

Of course, the battery management system is not enough. The temperature of the battery will increase during use. If the temperature is too high, the lithium-ion battery can no longer be used. This is an undesirable situation. As a result, the initial battery management system added thermal management capabilities. Later, it was found that the charge and discharge could not continue after the battery temperature was too low in a low temperature environment, and heating management was performed.

The use of the battery has been further expanded, and battery safety problems have increased, so there is a problem of safety management. The initial security management is monitoring. The BMS sends the battery data to the monitoring center. The monitoring center judges the security risk based on the data. Further development to the BMS itself to make an early warning of security.

The battery always needs to be maintained, replaced, balanced, etc. during use. These tasks require diagnosis. If the BMS has already diagnosed the data before it is needed, the data is ready, so the corresponding work will become much simpler. Therefore, the battery management system has added functions of fault diagnosis and reporting.

With the increase of decommissioned batteries, the use and recycling of the battery has shown problems. The use of batteries in the ladder requires a lot of research, and the BMS also assumes the management function of the optimized group.

The progress of battery research and development also depends on the problems and phenomena found in the battery use process, and depends on the choice of the actual use process, so the battery management system adds the function of battery technology selection.

Measurement is the basis of battery management, and more and more accurate, higher resolution technology is applied to battery management systems. The study of SOC estimation has also evolved from one-color Anshi integral to other methods such as Joule integral. There are more and more management functions of batteries, and it is worth noting that the rise of multi-level battery management systems.

From the master-slave structure to the development of each independent replacement unit can have a complete battery management system function. Beyond the battery system, vehicle battery management, and back-end server battery management procedures 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 an optimized use environment, and parameter derivation is optimized for use. As the industry develops, more and better battery management technologies and products can be expected.