Analysis of the dynamic behavior changes of supercapacitors during calendar life test under several voltages and temperatures conditions

Supercapacitors, also known as ultracapacitors are based on porous activated carbon electrodes and on electrostatic charge storage mechanisms. Carbon electrodes are supposed to be chemically and electrochemically inert and the electrostatic nature of the charge storage mechanism is highly reversible. These properties should assure that supercapacitors have an infinite shelf life. But in practice, supercapacitor cells exhibit performances fading when they are used for months. The purpose of this paper is to evaluate the performances fading of supercapacitors during calendar life test which correspond to lower current solicitations than power cycling test. In case of hybrid electric vehicle, which is a key application of ultracapacitors, this ageing test is useful because long rest periods represent a significant time of the vehicle real use. The degradation method of calendar life tests consists in maintaining the cells at high voltage and temperature. A periodic characterization based on impedance spectroscopy is done in order to quantify impedance changes. The obtained results confirm that impedance real part is increasing and the capacitance is decreasing.