Development of a Nonlinear Aging Model for Ultracapacitors

Ultracapacitors(UCs) are being increasingly used as an instant energy storage device due to their high power density, energy efficiency and almost instant recharging capability. Aging life of the UCs is an important area which is still need to be explored. The aim of our work is to develop a nonlinear aging model based on the impedance characteristics. The investigation has been done by coupling the aging model with the impedance based electric-thermal model of the UCs. The research is based on various literature that shows that the impedance characteristics strongly describe the UCs dynamic behavior. Electrochemical Impedance Spectroscopy (EIS) technique is used to investigate the impedance characteristics of the UCs. The parameters in the equivalent circuit model are identified using the least square method. The basis of this work is the experimental results that show the nonlinearity in the cell's capacity fade. The results show that the proposed nonlinear aging model provides better fitting than the linear aging model. In the near future this model can be used to for estimation and energy management in hybrid/electric cars.