Research On Estimation Algorithms Of Model Parameters And State-of-charge Of Lithium-ion Batteries Under Current Fluctuation

When the batteries are used in the microgrid,the energy storage converter may make the batteries charge and discharge frequently.At this time,the current fluctuation will have an impact on the performance of the batteries,and the estimation of the equivalent circuit parameters and state of charge(SOC)will become inaccurate.In this thesis,the lithium iron phosphate batteries are taken as the research object.In order to simulate different forms of current fluctuation,the bidirectional Buck-Boost converter is controlled.Estimation algorithms of the equivalent circuit parameters and SOC for lithium-ion batteries are studied to solve the problems of inaccurate parameter identification and SOC estimation caused by current fluctuation.The specific contents of this thesis are as follows:First of all,the types of current fluctuation are introduced,and the research status of the effect on the performance of lithium-ion batteries under low frequency and high frequency current fluctuations are introduced respectively,and the current states and progress about the methods of equivalent circuit parameter identification and SOC estimation for lithium-ion batteries is described.Secondly,the working principle of a bi-directional DC-DC converter for generating current fluctuation is analyzed in detail.The mathematical models of the bi-directional Buck-Boost converter in Boost and Buck modes are established respectively,and the generating principle of current fluctuation is analyzed.Thirdly,the equivalent circuit models of the batteries are introduced in detail.According to the circuit principle,the mathematical model of the second-order RC equivalent circuit of lithium-ion batteries is established,the nonlinear relationship between the open circuit voltage and the state of charge of the batteries is tested,and its mathematical expression is constructed.The basic principle of multiple adaptive forgetting factor recursive least square(MAFFRLS)is studied.and compared with the traditional forgetting factor recursive least square algorithm with only one forgetting factor.The experimental platform of the bi-directional DC-DC converter based on STM32F407ZG is built.The battery test on the special battery test device and the built experimental platform is conducted to verify the rationality and effectiveness of MAFFRLS.Finally,the square root cubature Kalman filter algorithm based on robust theory is presented to realize the accurate estimation of SOC when the current fluctuates.The estimation algorithm of SOC estimation for lithium-ion batteries is compared and verified under various conditions,it is verified that the SRCKF algorithm based on robust theory can improve the SOC estimation accuracy of lithium-ion batteries effectively.