False recovery? The impact of irregular rest periods on lithium-ion battery capacity and state-of-health estimation in intermittent use applications

Accurate State-of-Health (SOH) estimation underpins maintenance planning, warranty assessment, and safe operation of lithium-ion batteries, yet it rests on an assumption that fails in real-world intermittent use: that capacity fades smoothly and monotonically. This paper shows that irregular rest periods, characteristic of leisure vehicles, backup power, off-grid storage, and standby EV or grid assets, can trigger capacity recovery far larger and more persistent than the sub-1% transients reported in prior work. Drawing on a longitudinal dataset spanning over two years, thousands of cycles, and four lithium-ion cell types, we analyse the joint influence of C-rate, State-of-Charge during rest, and rest duration across the full ageing trajectory from fresh to advanced degradation. We observe recovery events large enough to alter the degradation pathway for hundreds of subsequent cycles, effectively masking a cell's true state and distorting cycle-life prediction. Critically, recovery magnitude is governed less by rest duration than by prior electrochemical imbalance and is most pronounced at the mid-range SOCs where systems are typically stored, a regime under-examined in existing literature. We demonstrate the consequences for SOH estimation using a Kalman-filter estimator applied to two contrasting cells: under sparse-update conditions typical of real-world BMS operation, tracking errors exceed 6% for a recovery-affected cell while remaining below 1.5% for one degrading monotonically. These findings establish rest-induced recovery as a first-order factor in degradation behaviour, not a measurement artefact, and motivate rest-aware or adaptive estimation methods for intermittently operated energy storage systems.