Parameters evolution of an ultracapacitor impedance model with ageing during power cycling tests

This paper deals with ageing evaluation of ultracapacitors (UC) during power cycling tests with high current levels. The quantification of ageing is made thanks to the monitoring of model parameters changes. The UC modeling uses a description of the main physical properties as porous nature of the electrodes. The model is defined by four main parameters which are identified thanks to specific characterization tests based on impedance spectroscopy. This model is validated for a new UC under different conditions of voltage and temperature. Then a power cycling methodology is applied. First, the specification of current profiles is made according to both power requirements in hybrid and electric vehicles applications and intrinsic properties of the UC. In order to have significant self heating, a current shape is chosen and used in a preliminary test allowing to verify that the thermal steady state is properly obtained. Then, the power cycling tests is started and the UC parameters changes are recorded thanks to periodic characterization tests. Finally, the results of the accelerated ageing are presented. They show two major effects that are opposed. The first one is the UC performances fading during power cycling tests and the second one is the performances recovery during the rest periods. The evolution of the impedance model parameters is presented and the link to physics is discussed.