Capacity-based state of health is an incomplete basis for deciding whether aged electric-ferry cells should remain in service or be reassigned to second-life use. This study reframes aged e-ferry cell evaluation as a sequential life cycle decision and extends earlier cell-to-cell analysis in the previous study by adding more diagnostic metrics and explicit cycle-to-cycle analysis from repeated complete cycle windows. A simulated daily ferry driving/charging profile is used for first-life extension, while low-rate and C/2 efficiency, voltage repeatability, apparent resistance, and 168 h rest behavior are used for second-life screening.

Defining matched start and end states for the complete cycle window is essential. The earlier retired 43 Ah M18 (cells from the module 18) C/2 efficiency below 80% is revised to 88.8-89.6%. Under the simulated ferry profile, M18 cells show 92.6-93.1% efficiency, while it is 96.9% for the 48 Ah retired M32/M33 cells and 98.0% for the 48 Ah calendar-aged MCA reference cells. Relative to M32/M33, M18 requires 44.3 kWh/MWh additional input energy, or 43.9 DKK/MWh, corresponding to about 121 kDKK/year for 2,750 MWh annual output. Cycle-to-cycle variation is low, as shown by small efficiency SD values and closely aligned voltage behavior. The main cell-group differences arise from efficiency level, rate sensitivity, and resistance-related voltage sag, where M18 cells show less favorable result. On the other hand, they show the best short-term capacity retention. Therefore, first-life and second-life decisions require multiple metrics rather than capacity alone.