On the requirements of the power converter for second-life lithium-ion batteries

The use of lithium-ion batteries is increasing year after year, especially in the automotive sector. Given the high requirements of electric vehicles, their energy storage systems are discarded when they still have around 70% of its initial capacity. These discarded batteries are being studied as a low-price option for stationary systems, mostly related to renewable energy generation, with lower battery requirements. However, the increasing dispersion of cell capacity detailed in this contribution limits the use of second-life cells if regular battery management systems and power converters. We present in this contribution an experimental comparison of the capacity dispersion between fresh and second-life cells, and detail the relationship between the capacity dispersion and the required BMS functionality. Furthermore, we include the ageing phenomena in the analysis by means of experimental ageing results, given that the capacity dispersion is enlarged as the battery ages. After this, we use this data to quantify advantages and disadvantages of a combined BMS and power converter, based on a multilevel topology, compared to a conventional BMS. The most relevant result, when a 55-cell battery is analysed, is a 65% increase in capacity during its whole second life if the BMS and power converter are combined by means of a multilevel topology. The increased level of complexity required by the combined BMS-power converter architecture is analysed in this contribution, providing a convenient tool for the selection of the most suitable option for each application.