Overview
To date, scientific results from studies on the ageing of on-board Energy Storage Systems (ESS), and particularly supercapacitors, are limited in number. The main reasons come from either the confidentiality of the results from the manufacturers, the limited number of technologies actually available and accessible to specialized laboratories, or because the studies do not consider the implementation of ageing laws in the developed models and therefore they are incomplete. However, in case of modern hybrid vehicle applications, the integration of ESS based on supercapacitors requires a realistic estimation of the lifetime of the cells for different profiles of solicitation.
This is essential for the optimal design of the ESS because the vehicle has to meet its initial specifications for a lifetime between 10 and 15 years. Moreover, for obvious reasons of cost, which is a criterion for the successful marketing of hybrid vehicles, we cannot rely on the initial oversizing of supercapacitor cells. In these conditions, it is essential to have accurate models in order to estimate the lifetime of supercapacitors for different modes of ageing, both in power cycling and calendar modes.
In this context, the project SIMSTOCK was launched in 2006 and concerns only the behaviour and the ageing in power cycling and does not take into account the calendar ageing. However, the average utilization rate of a particular vehicle is about 7000 hours for a 15 years lifetime. The calendar ageing appears as a dominant phenomenon and requires a specific research project on this topic as SUPERCAL. The organization of the project SUPERCAL is similar to the SIMSTOCK one takes benefits of both the experience and specific equipment of laboratories and industrial partners.
The main objective of SUPERCAL was to develop advanced models for the latest technology of supercapacitors in which the different ageing modes are taken into account. An essential part of this project focused on endurance tests where the voltage across the supercapacitor is kept constant by an external device whose role is to compensate the self-discharge of the cell. Under these conditions, the ageing was the result of a combined action of both the temperature and the voltage representative of the energetic state of the supercapacitor.
The monitoring of the characteristics evolution of the elements is achieved through continuous measurement of leakage current and thanks to periodic characterization tests based on capacitance, equivalent series resistance and impedance measurements. Thus, the calendar lifetime can be quantified thanks to evolution laws of these electrical parameters for fixed voltage and temperature conditions. More specific tests will be conducted with the aim to highlight and study other modes of calendar ageing, which correspond to real solicitations in hybrid vehicles applications. Firstly, from thermal point of view, tests will be conducted to study the impact of a slowly varying temperature profile on the calendar ageing, at a daily or weekly timescale. Then, another objective is to study the influence of the superposition of an alternative voltage component to the constant bias voltage on the changing characteristics of the samples. The link with the project SIMSTOCK and the inter-comparison of the results will be addressed through specific tests based on calendar ageing tests with intermittent cycling periods. Finally, all these results will be analysed in order to define behavioural laws for the different ageing modes and to implement them into simulation models.