Novel SOH Estimation Using the Differential Force Signal for Li-ion Batteries under Constraint Conditions

Accurate estimation of the State of Health (SOH) is crucial for ensuring the safety and reliability of lithium-ion batteries. Compared to conventional electrical signals, battery swelling behavior offers significant advantages as it contains richer aging-related information. This study investigates the aging characteristics of batteries under external mechanical constraints, and proposes an innovative SOH estimation method based on differential force (DFDV) analysis. Cycling tests were conducted on fully constrained LFP prismatic batteries under 0.2 MPa. Throughout the testing, both mechanical and electrical signals were synchronously monitored and recorded. The research systematically analyzes the swelling behavior and aging patterns of batteries. Through incremental capacity analysis (ICA), the aging behavior and underlying mechanisms under room-temperature and constrained conditions were revealed. Simultaneously, mechanical signal analysis demonstrated a strong correlation between mechanical characteristics and battery degradation. Based on the DFDV curves, a novel mechanical feature-based SOH prediction method is proposed. The study found that rapidly changing forces generate a new characteristic peak in the DFDV curves, whose positions and amplitudes exhibit strong linear relationships with SOH. This mechanical approach provides enhanced sensitivity to internal structural changes and degradation processes compared to traditional capacity-based methods. And we point out the superiority and application prospects of this method. This research provides a groundbreaking paradigm for in-situ safety monitoring of batteries using mechanical signals, offering substantial implications for enhancing early warning capabilities under abusive operating conditions and optimizing the safety design of battery systems in electric vehicles and energy storage application. Keywords Lithium-ion battery; State of Health (SOH); Differential Force; Swelling behavior; Mechanical constraint