Research On Intelligent Balancing Method Of Lithium-ion Battery For Electric Vehicles

Nowadays,with the sharp increase in energy demand and the serious pollution of the eco-logical environment,the development of clean and zero-emission electric vehicles has become an unstoppable trend,and traditional diesel and petrol vehicles will be gradually replaced.As an important energy source for pure electric vehicles and new energy-powered automobile vehicles,rechargeable lithium-ion batteries have the advantages of high energy density and long cycle life,occupying a dominant position in the power battery market in the electric vehicle industry.Due to the electrochemical characteristics of the lithium-ion battery materials,the voltage of a single cell is generally lower than the demanding working voltage.Therefore,a number of cells are usually seri-ally connected as a battery pack.However,the existing battery design and production technologies cannot guarantee the consistence of the parameters in different cells,such as the internal resistance,the aging speed,the degradation rate and the temperature distribution etc.The differences of these cell parameters could finally result in cell imbalance of cells in the same battery pack.Imbalance between the cells can reduce the performance and available capacity of the battery pack,and this situation may continue to increase during use,and may even pose a safety hazard,which severely limits the electric vehicle It is constantly intensifying during use and may even pose a safety hazard.These problems have severely restricted the rapid development of electric vehicles.In this paper,the cause of the inconsistency of the research object is analyzed for the lithium-ion battery pack of the vehicle.The corresponding equalization circuit structure is designed and the equalization sys-tem platform is built.The proposed equalization control algorithm is simulated and experimentally verified.The main research contents of this paper are as follows:(1)Based on the equivalent circuit of the Lithium-ion battery,a single cell model is established to describe the dynamic characteristics of the battery,the related characteristics of different equal-ization circuit structures are compared and analyzed.The bidirectional improved Cuk converter and isolated buck converter are used as the basic structure of the equalization circuit.Different lithium ion battery equalization system platforms are built for different application scenarios,simulation experiments are performed to confirm the parameters of the circuit structure.(2)A Lithium-ion battery equalization control algorithm is designed for the selected circuit struc-ture,qhich can make the state-of-charge of the battery pack be consistent.Considering the equilibrium speed and energy loss,the optimization target is set,and the relevant constraints of the battery system are combined to convert it into a global optimization problem with con-straints.The conjugate gradient method is used to solve the optimal solution of the equilibrium control.The feasibility of this control algorithm are verified by simulation results.(3)Since the battery equalization system is a nonlinear system,the above algorithm needs to solve the nonlinear optimization problem with constraints,fuzzy control method will be much faster in the solution speed,but the equilibrium speed may be very slow with the decrease of error in the later stage of equalization.In order to solve this problem,a variable universe fuzzy controller for Lithium-ion battery is proposed.The average voltage and voltage difference of adjacent batteries are selected as the system input,the universes of the input and output variables can change along with the change of the input variables.The results show that the designed Lithium-ion battery equalization fuzzy control method can effectively make the dif-ference between the state-of-charge of the battery converge to the allowable range in a short time.(4)The improved topology of the Lithium-ion battery equalization system is designed by multi-agent theory.The battery equalization system of the series Lithium-ion battery pack is equiva-lent to a multi-agent system in which a single battery is equivalent to an intelligent body(node),and the battery equalizer is equivalent to its connection side.Using the graph theory analysis,the battery equalization time decreases as the algebraic connectivity of the equalization sys-tem topology map increases.Simulation and experimental results show that adding only one equalizer in the traditional battery equalization system can greatly reduce the equalization time.(5)A multi-target charging control strategy for Lithium-ion batteries is proposed.A multi-module Lithium-ion battery charger based on an improved isolated buck converter is designed,in which the individual cells in the Lithium-ion battery pack can be separately charged by each convert-er module.Using the multi-module charger,an intelligent tracking charging control method is designed for the battery pack.Considering the requirements of user demand,battery bal-ance and temperature limit,the optimal battery charging current is planned by constructing and solving multi-objective optimization problems,and the planned charging current online adjust-ment strategy is designed to ensure that the Lithium-ion battery terminal voltage is always in its constraint.The control strategy based on passive theory is used to realize the actual charging current provided by the multi-module charger for the battery pack on the hardware platform to accurately track the preset charging current.The experimental results show that the designed multi-target charging control strategy for Lithium-ion batteries has a very good effect on battery balance while completing user requirements and temperature limits.