Consistency Intelligent Control And Low Temperature Flexible Preheating Of Lithium-ion Battery Pack

In recent years,with the implementation of the strategy of "carbon neutrality" and"carbon peak",new energy vehicles have developed rapidly.Lithium-ion batteries have become the main power source of new energy vehicles due to their high energy density,long service life and low self-discharge rate.It is expected that the global demand for lithium-ion batteries will reach 5 TWh in 2030.However,there are many scientific problems unresolved behind the rapid development.In particular,the theoretical methods of lithium-ion battery consistency intelligent control and low-temperature flexible preheating are far from mature,which restricts the healthy development of lithium-ion batteries in new energy vehicles and other industries.The current research mainly has the following two problems:on the one hand,there are many battery index parameters,which are coupled with each other and have complex relationships.It is difficult to comprehensively and quantitatively evaluate the consistency,resulting in poor consistency after battery grouping;Besides,due to the "short board effect",the battery needs to be balanced.The existing methods only balance voltage and SOC,and the long-time scale equalization strategy is missing.The quantitative relationship between different equalization targets is difficult to determine.On the other hand,the traditional low-frequency AC heating method requires additional power supply,which is difficult to be applied on vehicle,and the contradiction between preheating speed and battery damage is difficult to be solved.Therefore,it is of great significance to study the battery consistency control and low temperature preheating method to improve the available capacity,service life and economy of the battery pack and ensure the efficient and long-life operation of the power battery.For this purpose,this paper mainly carried out the following research on the above scientific problems:Firstly,the working principle and specification parameters of lithium-ion batteries are briefly introduced.The battery characteristic test platform is built and the test scheme is designed.The 18650 type ternary lithium and lithium iron phosphate batteries are taken as the research object.Based on high-quality experimental data,the inconsistency of lithium-ion batteries and the characteristic changes at different temperatures are analyzed in detail,which provides theoretical guidance for the subsequent research on lithium-ion battery consistency evaluation,equalization control and low-temperature preheating methods.Secondly,aiming at the problem of one-sidedness of single index evaluation before battery grouping,the advantages and disadvantages of common weighting and comprehensive evaluation methods are compared,and a comprehensive evaluation model based on coefficient of variation-TOPSIS consistency is established and verified.The results show that the model can quantitatively evaluate the comprehensive performance of the battery.After the comprehensive evaluation index is obtained,the batteries with similar index values are grouped.The test verifies that the consistency of the battery charge and discharge curve of the group after evaluation has been greatly improved,and the standard deviation of the charge and discharge voltage is reduced by more than 2 times compared with random combination category.Then,aiming at the problem of long-time scale inconsistency of battery,the SOH intelligent equilibrium control method based on game theory is proposed.By establishing the equilibrium game model of the battery,the profit function is determined,and the Nash equilibrium solution is obtained.The optimal charging and discharging strategy of each battery is obtained,and the long-time SOH equilibrium of the battery and the cooperative equilibrium of the battery SOC and SOH are realized.It can be seen from the simulation results that the charge and discharge strategy obtained by Nash equilibrium can greatly reduce the dispersion of battery SOC and SOH,realize the maximum utilization of battery pack capacity,solve the problem of inconsistency of battery SOH,make the same retirement time,and reduce the maintenance cost of lithium-ion battery.Finally,aiming at the problem of poor low temperature performance of lithium-ion battery,a low temperature flexible self-heating strategy of lithium-ion battery based on high gain controller is proposed.After the working principle of the new vehicle AC heater is analyzed in detail,its key parameters and stability are determined.Then the battery preheating test platform is built.The strategy can adjust the AC current according to the history and the expected heating rate.The AC heater generates a small AC current at low temperature and a large AC current at high temperature to heat the battery to a specific temperature.The experimental results show that the proposed method has strong robustness and high reliability under different control gains,heating cycles,initial switching frequency,ambient temperature,heating time and battery state,and has little effect on the state of health of lithium-ion battery.The contradiction between heating speed and battery damage is solved,and the flexible preheating of low-temperature battery is realized.In summary,in order to solve the problem of performance degradation caused by poor consistency and low temperature performance of lithium-ion batteries,this paper studies the three aspects of battery consistency evaluation,equalization control and low temperature preheating,and proposes a set of practical and efficient theoretical methods.It lays a solid foundation for improving the available capacity and low temperature performance of the battery,prolonging the service life of the battery and ensuring the efficient and long-life operation of the lithium-ion battery.