Repurposed batteries present unique challenges in cycle testing due to their prior usage history and intended second-life applications. Unlike new cells, these batteries exhibit different degradation patterns and require specialized evaluation protocols to determine residual capacity, predict modified degradation rates, and establish application-specific requalification criteria. The testing methodology must account for the battery's first-life conditions while verifying performance for its new operational requirements.

Residual capacity assessment forms the foundation of repurposed battery testing. Standard capacity measurements follow similar procedures as initial testing, but with critical modifications. A full discharge-charge cycle at C/3 rate provides the baseline capacity, but this must be repeated for at least three cycles to account for capacity recovery effects common in aged cells. The average of the last two cycles provides the operational capacity value. Testing temperature must be controlled at 25±2°C to ensure consistency, as thermal history significantly impacts aged batteries. Resistance measurements should accompany capacity tests, with both DC internal resistance and electrochemical impedance spectroscopy performed to characterize power capability degradation.

Degradation rate evaluation requires extended cycling under conditions mirroring the intended second-life application. A standard approach involves three-phase testing. Phase one consists of 100 cycles at C/2 discharge and C/3 charge rates to stabilize the cell. Phase two implements application-specific cycling profiles for 300-500 cycles, with capacity checks every 50 cycles. Phase three conducts accelerated aging at elevated temperature (typically 45°C) for another 100 cycles to project long-term behavior. The degradation rate is calculated from phase two data, excluding the initial stabilization cycles. Unlike new battery testing, repurposed cells often show nonlinear degradation patterns, requiring piecewise linear regression analysis across different cycle ranges.

Application-specific requalification criteria must consider both technical parameters and safety margins. For stationary storage applications, the key parameters are capacity retention (minimum 70% of rated second-life capacity) and round-trip efficiency (above 85% for most grid applications). Electric vehicle secondary use demands more stringent power capability requirements, typically less than 30% increase in DCIR compared to initial second-life measurements. All applications require safety verification including thermal runaway onset temperature testing, which should not decrease by more than 10°C from first-life values.

Modified testing protocols address several unique aspects of repurposed batteries. First-life history documentation is essential, including original chemistry, usage patterns, and storage conditions. Batteries without complete history require more extensive characterization. Depth-of-discharge swing testing evaluates capacity retention across different operating windows, crucial for applications not using full cycles. Calendar aging tests simulate intermittent usage patterns common in second-life scenarios, involving periodic cycling with extended rest periods between sessions.

Comparative analysis between first-life and second-life degradation reveals distinct patterns. Lithium-ion batteries typically show 2-3 times higher capacity fade per cycle in second-life applications compared to their initial use phase. The knee point phenomenon, where rapid degradation begins, often occurs at 20-30% earlier in cycle count for repurposed batteries. These differences necessitate modified end-of-life criteria, usually set at 60% of original capacity for most second-life uses instead of the 80% common in primary applications.

Safety testing protocols require enhancements for repurposed batteries. Mechanical integrity tests should include vibration and shock resistance evaluations even for stationary applications, as prior usage may have compromised structural components. Thermal stability assessment must include localized heating tests to identify potential weak points from previous operation. Gas evolution monitoring during cycling becomes critical, as aged batteries often exhibit increased outgassing rates.

Performance validation under partial state-of-charge conditions is particularly important for many second-life applications. Testing should include extended operation in the 30-70% state-of-charge range with periodic full cycles to assess balancing needs. Power capability at different states of charge must be verified, as repurposed batteries frequently show greater performance variation across the voltage range than new cells.

Data collection and analysis requirements exceed standard cycle testing protocols. Individual cell voltage monitoring becomes mandatory rather than optional, as cell-to-cell variation increases significantly in repurposed packs. Temperature monitoring should include at least three measurement points per cell or module to detect localized heating. Cycle data must be correlated with periodic reference performance tests conducted at standard conditions to separate operational effects from true degradation.

The interpretation of test results requires specialized approaches. Baseline drift correction accounts for capacity recovery effects during testing pauses. Data normalization should use the second-life rated capacity rather than original specifications. Statistical analysis must incorporate Weibull distribution methods instead of normal distribution assumptions due to the heterogeneous nature of aged battery populations.

Protocol implementation varies by battery chemistry and first-life history. Lithium iron phosphate batteries require different evaluation criteria than nickel-manganese-cobalt chemistries, particularly regarding voltage curve analysis for state-of-health assessment. Batteries from electric vehicle applications need different testing profiles than those from grid storage first-life use, reflecting their distinct aging patterns.

Standardization efforts for repurposed battery testing are emerging but not yet fully established. Current best practices recommend combining elements from industrial standards such as IEC 62660 with application-specific requirements. Testing programs should allocate additional time for characterization compared to new batteries, typically requiring 30-50% longer test durations to account for stabilization periods and more frequent performance verification cycles.

The ultimate goal of repurposed battery cycle testing is to verify three key parameters: remaining useful life under second-life conditions, safety performance throughout the operational window, and economic viability for the intended application. This requires balancing comprehensive evaluation with practical testing duration, leading to the development of accelerated protocols that maintain correlation with real-world performance while reducing validation time. The specialized nature of these protocols ensures that repurposed batteries meet reliability requirements without the excessive conservatism that would make second-life applications economically unfeasible.