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Electric vehicles, powered by electricity instead of fossil fuels, are the only long-term solution to the future of transportation. As the heart of electric vehicle, the power lithium battery system can be promoted smoothly only if it is fully understood. In this paper, from the perspective of the development trend of the important on-board power lithium batteries for electric vehicles at home and abroad, the more promising lithium ion batteries and their battery management system are compared and analyzed.
Unbalanced charging of lithium-ion battery pack rechargers can easily lead to overcharge and discharge problems, which seriously damage their service life. This paper proposes a new charging mode of intelligent charging machine, which makes the battery pack more safe and reliable to charge the charging machine, can prolong its service life, increase the safety, and reduce the use cost.
1 On-board lithium ion battery management system
As the monitoring brain of electric vehicle battery, the battery management system (BMS) in hybrid electric vehicle can realize the monitoring of the remaining battery power, predict the power intensity of the battery, and facilitate the understanding of the whole battery system and the control of the vehicle system.
In pure electric vehicles, BMS has intelligent adjustment functions such as predicting battery remaining power, predicting driving range and fault diagnosis. The use of BMS is particularly obvious for lithium-ion batteries, which can improve the service state of the battery, extend the life of the battery, and increase the safety of the battery. BMS will be a key technology for the future development of electric vehicles.
As shown in Figure 1, the data acquisition module in the BMS measures the voltage, current and temperature of the battery pack, and then transmits the collected data to the thermal management module and the safety management module respectively for data display. The thermal management module controls the temperature of the battery cell to ensure that the battery pack is in the optimal temperature range.
The safety management module judges the battery pack's voltage, current, temperature and SOC estimation results, and sends out fault alarm and takes emergency protection measures such as circuit break in time when a fault occurs. The state estimation module calculates SOC and health status (SOH) according to the collected battery status data.
At present, SOC estimation is important, and SOH estimation technology is not mature yet. The energy management module controls the charge and discharge process of the battery, including the balance management of battery power, which is used to eliminate the problem of inconsistent power of the battery pack. The data communication module adopts CAN communication mode to realize the communication between BMS and on-board equipment and non-on-board equipment.
The core functions of BMS are SOC estimation, equalization management and thermal management. In addition, BMS also has other functions such as charge and discharge management, charge management of pre-charger and so on. In the process of battery charging and discharging, management should be carried out according to the relevant parameters such as environmental state and battery state, and the optimal charging and discharging curve of the battery should be set, such as the charging current of the charger, the charging upper limit voltage of the charger and the discharging lower limit voltage of the charger. The capacitive load existing in the high-voltage system circuit of electric vehicles is equivalent to short circuit at the moment of power-on, so it is necessary to carry out the charging management of pre-charging machine to prevent the transient current impact on the high-voltage circuit.
2 Core functions of battery management system
2.1 SOC estimation
SOC is used to describe the remaining charge of the battery, which is one of the most important parameters in the battery life. SOC estimation is the basis to judge the battery overcharge and overdischarge. Accurate estimation can prevent the battery pack from overcharge and discharge to the maximum extent and make it run more reliably.
The estimation of battery SOC is highly nonlinear under the influence of the internal working environment and the external use environment. There are many internal and external factors that affect the battery capacity, such as battery temperature, battery life, battery internal resistance, etc. It is very difficult to accurately complete the SOC estimation.
Existing SOC estimation methods are as follows:
(1) ampere-hour measurement method. The ampere-hour measurement method does not consider the changes in the internal structure and state of the battery, so it has the advantages of simple structure and convenient operation, but the accuracy of this method is not high. If the current measurement accuracy is not high, then with the passage of time, the cumulative error of SOC will continue to increase, affecting the final results. This method is suitable for measuring the battery SOC in electric vehicles. If the measurement accuracy can be improved, it can be regarded as a simple and reliable method for measuring the SOC.
(2) open-circuit voltage method. The open circuit voltage of lithium-ion battery has an approximate linear relationship with SOC, which can be used to judge the internal state of the battery. However, due to the strict measurement requirements, the battery standing time should be at least 1h, which is not suitable for the online real-time detection of the battery in electric vehicles alone. In general, the open-circuit voltage method is often combined with the ampere-hour measurement method because of its high accuracy in the estimation of the value at the beginning and end of charging.
(3) Kalman filtering method. Due to its excellent error correction ability, Kalman filter method is especially suitable for hybrid lithium battery with severe current fluctuation. The disadvantage of this estimation method is that it requires high system processing speed.
(4) Neural network method. Neural network has the characteristics of distributed parallel processing, nonlinear mapping and adaptive learning, so it can be used to simulate the dynamic characteristics of batteries and estimate the SOC. However, this method requires a large number of reference data for the neural network to learn, and the data and training methods are required to be high, otherwise it will cause unacceptable errors.
2.2 Balanced management
In the process of making a battery, there are many processes, and differentiation will lead to inconsistency. The difference of the battery is important in the change of time and temperature, its internal resistance and capacity will be different. Large differences between monomers are more likely to cause overcharge or overdischarge, resulting in battery damage. Realizing battery balance can maximize the utility of power lithium battery, prolong the battery life, and increase the safety. At present, the mainstream equilibrium methods at home and abroad are as follows:
(1) resistance equalization method. This method is an important representative of energy dissipation equalization method. The method is simple and low-cost, but the energy loss is relatively large and the efficiency is low. It is only applicable to the system with low current charge and discharge.
(2) Switching capacitance method. This method is an important representative of non-dissipative equalization method, and it makes up for the shortcoming of resistance equalization. But its control circuit is complex, the balance speed is slow, the time is long, is not suitable for large current use.
(3) transformer equalization method. This method is an active equalization control method of series battery pack based on symmetric multi-winding transformer structure. Its disadvantage is that the circuit is complex, the device is many, the volume is too large, is not easy to expand the battery pack. It is generally used in charge and discharge of large current.
(4) Centralized equilibrium. The method can quickly transfer energy from the entire battery pack to the individual cells, and the volume of the centralized equalization module is smaller. However, the balancing operation of multiple batteries cannot be carried out in parallel and requires a large number of cable connections, which is not suitable for the battery pack with a large number of batteries.
2.3 Heat management
Temperature affects all aspects of battery performance. The inhomogeneity of temperature field will aggravate the inconsistency of battery pack, so it is necessary to manage it. The purpose of thermal management is to maintain the temperature of the battery system within a certain range through heating or cooling measures, and to maintain as much temperature consistency as possible within the battery pack.
Temperature management is important to complete the following four functions :(1) rapid heating of the battery pack under the condition of low resistance; (2) To ensure the uniform distribution of battery temperature field; (3) Accurate measurement and monitoring of battery temperature; (4) Effectively evacuate heat when the battery pack temperature is too high. Commonly used cooling methods include natural convection method, forced air convection method, liquid flow method, phase change material method and thermal management method, etc., commonly used heating methods include battery internal heating method, heating plate method, heating sleeve method and heat pump method.
Charging technology of lithium-ion battery charger
3.1 Current situation and development trend
In practical application, it is necessary to choose different charging modes according to the limitation of battery capacity to prolong the service life of the battery. There are many charging methods for lithium-ion battery chargers, among which the simplest is the constant voltage chargers. Lithium-ion battery pack is generally composed of a large number of monomers in series. Due to the difference in the manufacturing process of each monomer, there is inconsistency in internal resistance, voltage, capacity and temperature, which is easy to cause the imbalance in the process of charge and discharge, that is, the large capacity monomer is shallow, the small capacity monomer is overput, which will cause serious damage to the battery pack. The problem of unbalanced charge and discharge is the focus of research on lithium ion battery pack.
The charging technology requirements of battery chargers for electric vehicles include:
(1) The charging process of the charger is fast. Low specific energy of power lithium-ion batteries leads to short charging range of single-use rechargers, which has been an important factor limiting the development of electric vehicles. As long as the battery can be charged faster and more efficiently by the charger, the short range of electric vehicles can be indirectly remedied.
(2) Generalization of charger charging equipment. In order to pursue the relevant academic frontier, optimize their own products for as much as possible market share, a variety of new batteries emerge in an endless stream, and coexist in this market. In the case of different types and different voltage levels of batteries coexist, the charging equipment of the charger in public places should have a wider adaptability. On the one hand, the charger should be applicable to as many batteries as possible; on the other hand, the charger charger should meet the requirements of customers for different voltage levels.
(3) Intelligent charging strategy of the charger. In order to realize the non-destructive charging of the battery, monitor its charge and discharge status, prevent over discharge, and achieve the purpose of saving energy and delaying aging, a more intelligent charging strategy of the charger is needed. That is, different charging strategies are supplied for different batteries to match the charging curve of the battery charger.
(4) High efficiency of electric energy transformation. The energy consumption of electric vehicles is closely related to the operating cost. In order to further promote electric vehicles, it is necessary to balance their cost performance and reduce energy consumption as much as possible.
(5) Integration of the charging system of the charger. With system miniaturization and more functional requirements, as well as the improvement of reliability and stability, battery charger charging system and electric energy management system as a whole, integrated electric current detection and reverse discharge protection function, to achieve a smaller need for external components, integration of higher charger charging solution, Thus, the arrangement space is saved for the other parts of the electric vehicle, the system cost is greatly reduced, and the charging effect of the charger can be optimized and the battery life can be extended.
3.2 Charging technology of intelligent charger
Based on the above analysis of the current charging status of lithium ion battery pack and its charger, this paper summarizes a charging mode of intelligent charger based on electric vehicle BMS, aiming at the problems of unbalance and safety that are likely to occur in the charging process of lithium ion battery pack.
In the whole charging process of the charger, the BMS system is important for the monitoring of battery voltage and current signals, temperature and connection status of the lithium ion battery pack. The intelligent management system in the charger can monitor the output mode of the charging device in real time. Intelligent communication is realized between the BMS system and the intelligent management system of the charging equipment of the battery pack. Real-time mode comparison of the state of the battery pack and the charging equipment of the battery pack is carried out, and the optimal charging mode of the battery pack is selected.
In the initial charging process of the battery, BMS estimates the maximum allowable charging amount of the lithium-ion battery pack, that is, carries out SOC assessment on the monomer of the entire battery pack to measure the maximum chargeable amount of the battery pack. Combined with the pre-set safety factor of the charging amount of the charger, the maximum allowable charging amount of the battery pack is calculated.
During the charging process, the lithium ion battery pack is charged according to the maximum allowable charging amount of the charger. Make full use of the energy management module of BMS to control the charging balance of the battery pack to ensure the consistency of the parameters of the battery pack. At the same time, in the charging process of the charger, the SOC value should be detected periodically (the detection period is determined according to the new gradient of the battery charge).
Using the state estimation function of the BMS system, combined with safety management, the battery pack can be maximally prevented from overcharging. After reaching the maximum charging capacity of the battery pack, both the BMS and the intelligent management system of the charging device of the battery pack can intelligently control the charging controller of the battery pack and end the charging process of the battery pack. At the same time, the BMS disconnects the communication with the charging device monitoring system.
The charging mode of the intelligent charger can not only solve the problem of unbalanced charging of the lithium-ion battery pack charger, but also ensure the charging safety of the battery pack charger to the maximum extent, prolong the service life of the lithium-ion battery pack and ensure its use safety.
4 Lithium ion battery detection technology
China has vigorously developed the electric vehicle industry and actively promoted the construction of charging facilities for related chargers. However, many problems have been found in the operation of these demonstration devices, such as the screening and matching of batteries, the heating of the equipment, and the poor contact of the plug and unplug interface of the connecting device. If these problems can not be solved in a small number of devices, after a large number of applications of electric vehicles, there will be a situation of inundation, which is bound to have a negative impact on its development.
With the massive construction of electric vehicle infrastructure, there is an urgent need for related testing methods. Tianjin Electric Power Company carried out the project "Research on Testing Technology of Key Charging Equipment of Mobile Electric Vehicle Chargers", in which the most important test for the electrical changing station of electric vehicles was the test of battery pack.
Battery fault diagnosis, screening maintenance and charging technology based on BMS monitoring are important in electric vehicle electrical changing stations. The performance of battery screening devices and rechargers will be mainly tested. The research and mastery of the characteristics of lithium-ion batteries is conducive to the judgment of the accuracy of screening devices in electrical changing stations and the improvement of battery life.
Through the investigation of a large number of key charging equipment of chargers that have been put into operation, it is helpful to master their operating characteristics and fault characteristics, improve detection efficiency, and form a simple and quick mobile detection method. This will be a strong core technology support, conducive to the overall development of electric vehicles.