Second Life Assessment of Salvaged 18650 Lithium-Ion Cells Using Capacity and Internal Resistance Metrics

The increasing waste stream of end-of-first-life lithium-ion batteries poses an important challenge and a circular economy opportunity via second-life applications. Nonetheless, trusted characterisation strategies are critical for confirming the safety and functionality of repurposed cells. This study analyses the viability of 18650 cells salvaged from laptop, power tool, and electric vehicle (EV) battery packs according to a dual-parameter framework based on residual capacity (State of Health, SoH) and DC internal resistance (DCIR). Measured SoH ranged from 50.0% to 98.0%, and DCIR ranged from 90 to 500 mΩ across the 22 characterised cells. This is, to our knowledge, among the first studies to combine dual-parameter screening with an explicit threshold sensitivity analysis across three distinct salvaged cell chemistries. Although 77.3% of the 22 characterised cells continued to possess a SoH ≥ 70%, only one cell (4.5%) passed a conservative reuse threshold in which both SoH ≥ 70% and the direct current internal resistance (DCIR) were < 100 mΩ. SoH showed a strong inverse correlation with DCIR (Pearson r = −0.76, R² = 0.57). Cells from the EV system showed maximum SoH retention with fair DCIR readings. The findings show that internal resistance, rather than capacity fade alone, is a key limiting factor to second-life applicability for power-demanding uses and highlight the need for multi-parameter screening protocols including resistance to reduce risks of unwanted low-performing battery reuse.