Rest-Free State-of-Charge Algorithm for Lithium-Ion Batteries Under Wide C-rate Conditions for Unmanned Underwater Vehicles

This paper presents a state of charge (SOC) estimation algorithm designed for unmanned underwater vehicles (UUVs) that operate continuously under low current conditions with intermittent high current phases from propulsion and fuel cell (FC) charging. The algorithm combines terminal voltage estimation with internal resistance compensation during low current operation and applies coulomb counting in high current events to minimize voltage-induced errors. A polarization voltage relaxation mechanism enables accurate transition back to voltage-based estimation. Experimental validation using a 72 [Ah] lithium-ion pouch cell under simulated UUV load profiles demonstrated a maximum SOC error of 3.00% and a final error of 1.80%, reducing long-term drift compared with conventional coulomb counting (5.98%). The results confirm that the proposed method maintains high SOC accuracy in endurance-critical missions and offers a reliable approach for efficient UUV battery management.