The expanding market for second-life lithium-ion batteries, which find their utility in residential energy storage among other applications, emphasizes the necessity for an efficient classification system. This system needs to address not only the potential for reusability but also the recyclability and safety protocols requisite for handling aged or deteriorated battery cells and packs.
The State of Usability (SOU), proposed by Wett C et al., emerges as a pioneering metric in this context, providing a consolidated indicator of a battery's viability after its primary life cycle. While indicators like the state of health (SOH), state of power (SOP), and others provide an understanding of a battery's current state, they fall short in predicting the battery's second-life applicability and recyclability. The concept of a State of Usability (SOU) fills this gap, serving as a critical tool for automotive engineers, recycling companies, and emergency responders in determining the future course of action for used batteries.
The SOU is derived by examining batteries within three principal categories: 'second life usability, recyclability, and limited recyclability' necessitating safe handling. This categorization is essential, as batteries might still retain significant functionality even after reaching their defined end-of-life (EOL). The SOU, therefore, offers a more granular and operationally relevant classification of battery states, especially when considering their potential for second-life applications or the imperative of safe recycling processes.
The decision tree displayed in the image functions as a practical tool for the classification of the SOU into 5 distinct levels. Beginning with an assessment of visible mechanical damage, the decision tree traverses through various binary conditions such as thermal runaway, electrolyte leakage, corrosion, and the status of the current interrupt device (CID). Based on the outcomes, the battery is classified into one of five SOU ranges. For instance, a battery with no visible damage, a closed CID, no overcharge or deep discharge incidents, and an SOH and SOP greater than 0.8, is considered fully usable for second life applications, thus falling into the highest SOU category of 0.8-1.
The methodology for the calculation of SOU for second-life applications involves a formulaic approach that considers the defects and aging mechanisms of the battery. This method allows for the determination of the SOU in a continuous range, especially applicable within the top two usability levels, providing a more refined understanding of a battery's second-life potential.
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