Calendar aging represents a critical factor in determining the viability of batteries for second-life applications. Unlike cycle aging, which results from repeated charge-discharge events, calendar aging occurs due to chemical and electrochemical degradation mechanisms during storage or idle periods. The implications of calendar aging become particularly significant when evaluating retired electric vehicle (EV) batteries for repurposing in stationary storage systems, where remaining useful life must be accurately assessed to ensure performance and safety.
The extent of calendar aging depends on multiple factors, including prior usage conditions, state of charge (SOC) during storage, temperature exposure, and time duration. Lithium-ion batteries, the most common chemistry considered for second-life use, experience accelerated degradation when stored at high SOC levels and elevated temperatures. The formation and growth of the solid electrolyte interphase (SEI) layer on the anode, electrolyte decomposition, and transition metal dissolution in the cathode contribute to capacity fade and impedance rise over time. These mechanisms are often irreversible and accumulate even when the battery is not in active use.
Assessment methodologies for repurposed batteries must account for prior calendar aging effects to predict remaining useful life accurately. A multi-stage evaluation process typically includes initial screening, performance testing, and degradation modeling. Initial screening involves checking historical data logs from the battery management system (BMS) to identify storage conditions, average SOC, and temperature exposure during the first life. Batteries that spent extended periods at high SOC or in hot climates may exhibit more severe calendar aging, reducing their suitability for second-life applications.
Performance testing focuses on measuring current capacity, impedance, and self-discharge rates. Capacity measurements at different discharge rates provide insight into remaining energy storage capability, while electrochemical impedance spectroscopy (EIS) helps identify increased internal resistance due to SEI growth or other degradation mechanisms. Elevated self-discharge rates may indicate micro-shorts or electrolyte breakdown, which could compromise long-term reliability. These tests are often supplemented with accelerated aging protocols that simulate years of additional calendar aging in a compressed timeframe, helping project future degradation trajectories.
Degradation modeling combines empirical data with physics-based approaches to estimate remaining useful life. Empirical models rely on statistical correlations between measured parameters and known aging behaviors, while physics-based models incorporate fundamental electrochemical principles to predict how prior degradation will influence future performance. Hybrid approaches that merge both methods have shown improved accuracy in accounting for calendar aging effects. Special attention must be paid to the nonlinear nature of calendar aging, where degradation rates often accelerate as the battery ages.
Second-life applications present varying demands that influence how calendar aging impacts performance. Grid-scale energy storage systems typically operate at moderate SOC levels with controlled temperature environments, potentially slowing further calendar aging. However, these systems require long service lives, meaning even small annual degradation rates can become significant over time. Residential energy storage systems (ESS) may experience wider temperature fluctuations and more frequent cycling, creating complex interactions between calendar and cycle aging. The assessment process must consider these usage patterns when evaluating whether a retired battery can meet the expected service duration.
Aerospace and marine applications introduce additional challenges due to their demanding operating environments. Batteries destined for these uses require more stringent screening to account for prior calendar aging, as the combination of high performance requirements and limited maintenance opportunities leaves little margin for degradation. Medical device applications demand exceptionally high reliability, often necessitating conservative estimates of remaining useful life that incorporate safety buffers based on calendar aging history.
Several technical approaches have emerged to mitigate calendar aging effects in second-life batteries. Active cell balancing can help equalize SOC across modules during storage, preventing individual cells from remaining at high voltage states. Adaptive charging algorithms that optimize SOC based on anticipated idle periods can reduce degradation rates in the second life. Thermal management systems become especially important for repurposed batteries, as they help maintain optimal temperature ranges that minimize further calendar aging.
The development of standardized assessment protocols remains an ongoing challenge in the second-life battery industry. Without consistent methods for evaluating calendar aging effects, comparisons between different battery batches become difficult. Some organizations have proposed tiered classification systems that group batteries based on their remaining capacity and impedance characteristics, with more stringent requirements for applications needing longer service lives. These classification systems often incorporate calendar aging considerations by adjusting thresholds based on storage history.
Material-level differences also influence how calendar aging affects second-life potential. Batteries with nickel-manganese-cobalt (NMC) cathodes may exhibit different calendar aging behaviors compared to those with lithium iron phosphate (LFP) chemistry. NMC batteries typically show stronger voltage dependence in their calendar aging rates, while LFP batteries are more sensitive to temperature variations. These material-specific characteristics must be factored into assessment methodologies to ensure accurate predictions of remaining useful life.
Ongoing research continues to improve understanding of calendar aging mechanisms and their implications for second-life applications. Advanced diagnostic techniques, including post-mortem analysis of retired batteries, have revealed how microstructural changes during calendar aging influence long-term performance. This growing knowledge base supports the development of more sophisticated assessment tools that can better account for the complex interplay between prior usage history and future degradation patterns.
As the second-life battery market matures, the ability to accurately evaluate and manage calendar aging effects will become increasingly important. Robust assessment methodologies that incorporate comprehensive testing, advanced modeling, and application-specific considerations will be essential for ensuring the reliable performance of repurposed battery systems across various use cases. The successful integration of calendar aging analysis into second-life battery evaluation processes will support the sustainable extension of battery service life while maintaining necessary performance and safety standards.