Research On Direct Balancing Topology And Path Optimization Strategy For Series Connected Batteries

As the environmental issues increase,the development and use of clean energy has attracted more and more attention.There are many types of rechargeable batteries available on the market.Lithium-ion batteries are considered to have the best application prospects in all battery types due to their superior characteristics,performance,positive environmental impact and recovery potential.The phenomenon of overcharging and overdischarging of some battery during repeated use cycles is an important reason for the reduction of effective capacity and cycling life of the battery pack,and it becomes a bottleneck restricting the safety and reliable operation of the battery pack.Therefore,various types of switching-based energy converters raise.Active balancing technology has become a research hotspot at present.In this paper,the existing switching based technology on the principle of balancing topology and scope of application are analyzed.The series lithium-ion battery pack is considered as the research object,with the aim of increasing the equalization speed and simplifying the struture.Then the study on active equalization circuits,the system structure and the path optimization strategy are carried out.A graph-based quantitative analysis method for the balancing system structure is proposed.Firstly,the series connected battery pack is modeled by using the directed graph and the weighted accessibility matrix to obtain the characteristics of the ideal balancing unit sturcture,i.e.the completeness and bi-direction.The balancing efficiency and speed of different types of systems are studied.The ideal equalization structure derived above is used as the basic balancing unit model,and three large-scale balancing system structures of single-layer,chain,and multi-layer are established.Then,a hybrid balancing system structure is proposed.The impact of the number of batteires on the equalization efficiency and speed in each structure is analyzed.Also,the differences between the equalization efficiency and speed of different balancing systems are compared.Then,the reasonable balancing system structure is summarized.The lower layer direct balancing topology is based on flying inductor and the upper layer based on multi-inductive coupled topology is proposed.Double-layer hybrid equalization system based on two structures is established.Firstly,the energy transfermation among the four operational modes of the lower-level cell direct equalization topology is analyzed,including any cell to cell,any cell to the adjacent connected cells,any adjacent connected cells to any cell and any adjacent connected cells to other adjacent connected cells balancing.For different imbalanced conditions,the balancing topology has multiple balancing paths,and fewer energy conversion times in the equalization process.At the same time,the experiment verifies that the balancing topology can raise energy transferred and improve the speed of the equalization.Secondly,the energy transfermation between the two modes of operation of the upper-level unit's direct equalization topology is analyzed,including the forward mode and the flyback mode.For different imbalance d conditions,the balancing topology can achieve fast equalization,and energy can be delivered in a spanning manner,no energy overlap,short paths,and fast speed.For the double-layer hybrid equalization system,the optimal strategy of the system equalization path is established,and the principle of the optimal efficiency or the optimal speed under the complex imbalanced condition of the long-series battery pack is studied.Firstly,the equalization efficiency and speed of the hybrid balancing system are modeled,and the corresponding constraints are determined.Then the ant colony algorithm is used to solve the proposed cost function.Finally,this strategy is applied to a 13-series-battery-cell system and a 97-series-battery-cell system respectively.The simulation verifies that the proposed path optimization control strategy can quickly plan the equalization path when dealing with the complex imbalanced conditions of the long-transmitted series battery pack.A cell direct equalizer based on the improved flying inductance is used as a lower basic equalization unit,and a multi-inductor coupled unit direct equalizer is used as the second-layer equalization unit to construct a dual-layer hybrid equalization system.The equalization experiments are performed under the static,charging and discharging conditions of the upper and lower level equalizers.Experimental results show that the two direct equalization topologies proposed in this paper can achieve effective equalization with fewer energy conversion times and faster speed.At the same time,the influencing factors of the average balancing current of the upper and lower equalizers and the equalizer efficiency are verified by experiments.Finally,a balancing experiment is conducted with the double-level equalizers working simultaneously.The experimental results show that the double-layer equalizer works at the same time can maximize power transmission,with a short balancing path,and fast equalization speed.