Orbit-Coupled Multiphysics Degradation Modelling of CubeSat Lithium-Ion Batteries

CubeSats operate under severe orbital thermal cycling and limited thermal control authority, making lithium-ion battery degradation a primary mission-limiting factor. Repeated sunlight–eclipse transitions in low Earth orbit induce high-frequency temperature oscillations, while progressive internal resistance growth amplifies Joule heating over multi-year operation. Existing degradation models typically treat electrochemical aging and thermal behaviour independently, neglecting cumulative orbit-coupled feedback mechanisms that drive nonlinear lifetime reduction. This study presents a Multiphysics, orbit-resolved cumulative degradation framework that integrates Arrhenius temperature dependence, current-induced electrochemical stress, resistance growth, and transient thermal balance within a unified differential formulation. The model captures progressive coupling between heat generation, temperature rise, and degradation kinetics across mission-scale timescales. Results demonstrate pronounced nonlinear capacity fade, convex resistance acceleration near end-of-life, and thermal amplification effects that substantially reduce predicted mission duration. Sensitivity analysis identifies activation energy and orbital current amplitude as dominant reliability drivers. The proposed framework enables physics-consistent mission lifetime prediction and provides a quantitative design tool for CubeSat power system optimization, reliability assessment, and thermal margin evaluation under realistic orbital forcing conditions.