A Monotone Rational–Quadratic Spline Driven High‐Fidelity Open Circuit Voltage Modeling for Lithium‐Ion Batteries

ABSTRACT Reliable state estimation of Li‐ion batteries in energy applications depends largely on accurate modeling of the open‐circuit voltage (OCV), denoted by V oc , as a function of state of charge (SoC). The V oc –SoC characteristic is conventionally represented using high‐order polynomials or lookup tables stored in the battery management system (BMS). High‐order polynomial models may suffer from numerical issues and generally require numerical root‐finding for inverse mapping, whereas lookup‐table approaches may require a dense set of breakpoints, leading to increased memory usage. To address these limitations, this article proposes a monotone rational–quadratic spline (RQS) framework for V oc –SoC modeling. A greedy knot‐selection algorithm is incorporated into the RQS framework to adaptively select breakpoints along the V oc –SoC characteristic. The proposed approach is validated on test data from six Li‐ion cells with different chemistries, collected at multiple operating temperatures. The proposed method provides a smooth and monotonic representation with analytical invertibility and low computational complexity compared to conventional models. The efficient pseudo‐ V oc ‐based SoC reconstruction suggests the proposed framework's potential suitability for integration into real‐time BMS applications.