Since EV batteries lifetime is about 8-10 years, it has been estimated around a million batteries retired from EV applications in 2030 in the EU. The end-of-life management of EV batteries is currently regulated by the EU Battery Directive, and decommissioned EV batteries are 100% collected. As these end-of-1 st life batteries may still have up to 80% of their initial capacity, it was estimated that in Europe 2nd life battery, when used for stationary application, would reach 80GWh/year by 2030. Repurposing this attractive large volume of batteries faces however some challenges like the trending price drop of the new batteries, the big spread in the EVs battery pack design, and the lack of second life quality and performance standards.
The PhD work will focus on the modelling and control of battery packs composed of 2nd life Li-ion batteries to be used in stationary applications. Such the battery pack might be constituted of modules from different technologies and subjected to considerable state-of-health (SOH) imbalances. First, a model of the pack will be developed, accounting for the pack topology (cell-to-cell electrical interconnections) as well as thermal coupling between cells. Second, the model will be used to derive a state estimator in order to infer the internal state of each battery cell, in terms of notably state-of-charge (SOC) and SOH. Since battery cells with different health conditions are considered, a diagnosis system is also envisioned, which will be responsible to detect critical faulty cells and isolate them from the system. The SOC & SOH estimates will be then exploited in a control loop in order to properly charge and discharge the cells for the mitigation of battery pack degradation in the long term. This approach will ensure an improved performance of the battery pack throughout its remaining useful life compared to a system where no feedback action is executed. This will enable the repurposing of retired EVs battery pack while ensuring an optimum performances and useful life of battery systems that are of different qualities and are at different aging stages without compromising the performance of the overall system.
