| In recent years,with the booming development of the new energy vehicle industry,the scale of retired power batteries for electric vehicles has increased dramatically.Composing energy storage system by using retired batteries can extend battery life and improve resource utilization.However,the retired batteries have inconsistent capacity and SOC due to long-term charging and discharging cycles,and the inconsistency is more obvious when the number of series-connected batteries is large.In order to make full use of the available capacity of the secondary use battery pack,it should be balanced to improve the capacity utilization of the battery pack.In this paper,we take the energy storage system of the secondary use battery as the research object and study for its equalization system.The main contents are as follows.Firstly,the relationship between the capacity difference of the secondary use battery and the equalization current is analyzed,and the synchronous current control equalization unit is adopted according to the requirements of independent and synchronous control of the equalization current,and real-time high current equalization.Through mathematical model and theoretical derivation,the working principle and control method of the equalization unit,the principle of MOSFET overvoltage and the design of absorption circuit parameters are analyzed in detail.The equalization unit has the capability of synchronous current control,and through the simultaneous control of multiple MOSFETs,the equalization current of each cell can be controlled independently.The equalization unit has fewer switching devices and a simple structure,and provides a larger equalization current for each battery in real time,with a faster equalization speed and strong scalability.Secondly,in order to fully equalize all batteries to improve capacity utilization,and in view of the large number of batteries and high voltage levels in the energy storage system of secondary use batteries,a multi-stage equalization system based on synchronous current controlled equalization unit is designed.The multi-stage equalization system can effectively reduce the voltage stress of switching devices inside the equalization unit and reduce the system cost and losses.The cost and benefit advantages of multi-stage equalization technology are further illustrated by comparing the economics of various forms of energy storage,which can efficiently equalize the entire battery pack.The steady-state and dynamic control performance of the multi-stage equalization circuit is verified through simulation.Thirdly,with SOC consistency as the equalization control objective,the equalization unit control strategy based on capacity inconsistency is proposed and a multi-stage SOC equalization control strategy is designed.To improve the SOC estimation accuracy and SOC equalization effect,the SOC equalization strategy is further optimized by designing online SOC correction strategy,online capacity estimation and correction strategy,and the SOC equalization control and its optimization strategy are verified by simulation.Based on the above functions of the multi-stage equalization system,the integrated inspection-operation-maintenance control strategy is designed to reduce the labor input and time cost during the retired batteries screening,energy system operation and maintenance.Finally,the hardware and software design of the multi-stage equalization system is introduced in detail,and a prototype multi-stage equalization system is built for a battery unit composed of 12 series-connected cells.On the experimental platform,the steadystate and dynamic performance of each stage synchronous equalization unit is verified to prove the synchronous current control capability of the equalization unit;the multi-stage equalization system is experimented,and it is proved that the equalization units at each stage can work independently and operate synchronously without affecting each other;then,on this basis,the multi-stage SOC equalization experiment of the batteries is carried out,and the effectiveness and feasibility of the proposed multi-stage SOC equalization technology are verified by analyzing the experimental results and data. |
