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An accurate estimation of the residual energy, i. e., State of Energy (SoE), for lithium‐ion batteries is crucial for battery diagnostics since it relates to the remaining driving range of battery electric vehicles. Unlike the State of Charge, which solely reflects the charge, the SoE can feasibly estimate residual energy. The existing literature predominantly focuses on showcasing diverse methods with a gap in conducting in‐depth analysis and comparison of the SoE. The scope of this work is to provide a comprehensive understanding of the SoE by discussing the feasibility and applicability of various definitions and estimation approaches from the literature. For the first time, we classify existing SoE definitions, considering the differences between the inherent stored and usable energy. In the absence of a unified definition in the literature, we propose two physically feasible definitions. Based on the proposed definitions, we critically analyze the practical challenges of SoE estimation. Additionally, to examine the accuracy of the most suitable SoE definition, we conduct an experimental evaluation using a commercial NCA/C+Si cell. Compared to the State of stored Energy, we demonstrate that the State of Charge underestimates the residual energy by more than 3 %, highlighting the importance of adequate SoE estimation.

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