Walid Lajnef

Power cycling tests for accelerated ageing of ultracapacitors

In this paper, a test methodology is presented and evaluated in order to quantify the lifetime of ultracapacitors in case of active power cycling. The test profiles are specified for taking into account both the high charge-discharge levels and the periodicity of typical HEV applications. Before cycling, preliminary tests have been done with Maxwell BCap 2600F devices and have proved that pulsed current profiles can be predetermined in order to induce a given self-heating. Then, during power cycling, periodic characterization tests have been done in order to follow the evolution of cell parameters. The results at 25,000 cycles illustrate that impedance real part and capacitance are significant parameters for ageing quantification. Furthermore, the trend of these results shows that the self-heating acts as an accelerating factor for ageing.

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.

Monitoring fading rate of ultracapacitors using online characterization during power cycling

This paper deals with the ageing quantification of ultracapacitors in cycle-life tests. Typical micro- and mild-hybrid applications have been used to specify the test profiles in agreement with power requirements. Since ultracapacitors lifetime depends on the temperature, these profiles are adjusted to induce a given self-heating at the beginning of the power cycling tests. Ageing is evaluated thanks to the measurements of the electrical parameters changes. The good reproducibility of the characterization methods has allowed us to monitor the fading rate of ultracapacitors, from two manufacturers, with the proposed current profiles.