# A Strategic Pathway from Cell to Pack-Level Battery Lifetime Model Development
###### Abstract
Perhaps, physics-based modeling could solve the puzzle of monitoring battery state of health (SoH), and if linked with data-driven techniques, can precisely forecast the system failure to avoid unprecedented colossal damage [20; 21]. Thus, Horizon Europe has identified this topic as one of the future scopes of research for batteries in Europe [22]. Moreover, the electrical and thermal properties of the battery pack directly impact battery aging, thus being the dominant parameters in the lifetime model. The SoC prediction and thermal simulations are usually done at the pack-level to understand the concerning behaviors; however, it is rare to be involved in the aging contribution, which should be the case [23]. Thus, an efficient electro-thermal coupling framework is proposed to be linked to the aging model. Hence, the impact of an efficient thermal management system (TMS) is undeniable as both the cooling and heating service positively improve the battery pack health. Researchers are continuously developing better thermal management methodologies and materials to extend the pack lifetime; however, that is not the aim of this research [24; 25]. Generally, 1D electro-thermal models benefit from easy integration into system-level frameworks (e.g., an entire EV model) and fast simulation runtimes, while they provide an acceptable averaged surface temperature of the battery cells [26]. The baseline of the electro-thermal models can start from an equivalent circuit model [27] or atomistic level entailing battery physical-chemical phenomena in a multi-scale simulation environment [28]. On the other hand, 3D thermal models provide higher accuracy in terms of temperature values and temperature distribution patterns alongside every point of the battery geometry, e.g., tabs, sides, inner part, etc. [29; 30]. These features help determine the hottest and coolest points of the batteries for specific applications for a proper thermal management design. Yet, they require much more computation resources for the computational fluid dynamics (CFD) solver leading to high CPU-to-real-time ratios (i.e., slow simulation runtimes), and they are not inherently implementable into a 1D system-level modeling framework for integration. Researchers have tried to gather the benefits of both model types by replacing the thermal part of the 1D electro-thermal model [31] with a reduced-order model of the 3D CFD battery model for temperature calculations (for quick computation), while the electrical properties of the battery are achieved by the 1D electrical part [32]. Thus, an optimal, robust, and precise electro-thermal model can be an integral part of the whole modeling framework where lifetime, safety, and reliability sections work together providing feedback to each other [33]. The proposed pathway is focused on battery aging that is kicked off with commercial cells' (LTO technology) cycle life study to generate a high-quality dataset. The detailed investigation results in a thorough understanding of battery aging that leads to the development of a robust empirical lifetime model. The model is validated to a realistic standard profile before being extended to the pack-level following an approved design that has been demonstrated as part of Horizon2020 European project GHOST [34]. The augmented lifetime model employs an integrated electro-thermal system consideration while simulating the lifetime. This coupled 1D electrical and 3D thermal model is developed and validated within the framework of another Horizon2020 project (Vision xEV) [35]. Upscaling the pack-level electro-thermal simulation result is generated using commercial AVL advanced simulation technologies (AST) packages and linked to the lifetime model appraising the actual health condition in the remaining life calculation. Hence, the rationality and applicability of such models in the BMS for online use cases are discussed providing perspective-based guidance to the research community.