Research On Performance And SOC Estimation Of Lithium-ion Capacitors

Pure electric vehicles(PEVs)are gradually replacing traditional fuel vehicles as the best choice for citizens to travel because of their energy saving,low noise and other advantages.The new lithium-ion capacitors(LIC)are widely used in cities due to high power density and they can also meet the energy density requirements of urban working conditions.The new lithium-ion capacitors realize the organic combination of lithium-ion battery and super-capacitor to the maximum extent,and has both performance advantages.However,up to now,the main way to judge when the city bus carrying new lithium-ion capacitors is full or when the city bus should be charged is still based on voltage empirical threshold,which can not estimate the state of charge(SOC)accurately.The inaccurate estimation of SOC will lead to the reduction of battery-capacitor utilization,energy waste,driver's misjudgment and even the potential safety hazard to passengers.Therefore,it is important to develop a stable and suitable SOC estimation method for battery-capacitor management system(B-CMS).The in-depth understanding of the basic performance of lithium-ion capacitors is a prerequisite for the development of SOC algorithm.In this paper,the experiment of ratio and temperature characteristic is carried out,and the law of capacity changing with current and temperature is qualitatively analyzed;The open circuit voltage(OCV)characteristic experiment at different temperatures is designed,and the result shows that OCV is sensitive to temperature;The experiment of self-discharge rate is designed,and the self-discharge phenomenon after shelving is analyzed;The HPPC characteristic test at different temperatures is carried out,and the change of internal resistance and OCV with temperature is analyzed.Aiming at whether the existing SOC estimation methods for lithium-ion batteries are suitable for the lithium-ion capacitors,the SOC estimation method based on parametric estimation OCV and two SOC estimation methods based on OCV-Ah counting are studied respectively.Among them,the first-order RC equivalent circuit model is selected to track the dynamic behavior of battery-capacitor and the relational model between dynamic OCV and SOC is constructed.For the SOC estimation based on parametric estimation OCV,this paper selects the recursive least squares method with forgetting factor for parameter identification.The maximum absolute error is 2.73% and the root mean square error is 0.70%;For the SOC estimation based on OCV-Ah counting,this paper selects the extended Kalman filter(EKF)method,which combines model parameter identification with SOC estimation process.The maximum absolute error is 0.93% and the root mean square error is 0.47%;At the same time,a simplified EKF method is used to estimate the SOC of battery-capacitor with the maximum absolute error of 1.38% and the root mean square error of 0.70%.Aiming at the problem that the OCV of new lithium-ion capacitors is affected by temperature and causing the SOC estimation deviation at low temperature,this paper corrects the dynamic OCV expression at room temperature and establishes the threedimensional mapping model of OCV-SOC-T.When the model is at-10?,the maximum absolute error of SOC estimation is 4.68%,and the root mean square error is 3.38%.Compared with the uncorrected dynamic OCV-SOC model,the maximum absolute error is reduced by 3.78% and the root mean square error is reduced by 2.78%.Based on the threedimensional mapping model of OCV-SOC-T,three SOC estimation methods are validated and evaluated in terms of accuracy,computational complexity and convergence speed at different temperatures.Finally,the simplified EKF method is chosen as the SOC estimation scheme in B-CMS.In order to realize the application of SOC estimation method in vehicle,B-CMS is developed in this paper.Based on the requirement analysis of the system,the overall framework of B-CMS is designed,and the components selection and circuit design of each functional unit are carried out.The accuracy of voltage and current acquisition of B-CMS is analyzed under DST condition,and the reliability and accuracy of the SOC estimation method for B-CMS determined in this paper are verified.