The Promise of Second-Life Batteries

As electric vehicles proliferate, so too does the number of lithium‑ion battery packs approaching the end of their automotive service. Although a pack may no longer deliver the power or range required for vehicular use once its capacity dips below roughly 80% of its original rating, substantial energy storage potential remains. Repurposing these “retired” packs for stationary applications—such as home energy storage or grid balancing—can extend their useful life by years, reducing waste and improving the economics of renewable energy integration. Evaluating whether a used battery is suitable for second‑life deployment demands an understanding of how its past usage has eroded capacity and how quickly further degradation may progress.

Degradation Mechanisms

Lithium‑ion capacity fade arises from both cycle aging and calendar aging. Each charge‑discharge cycle consumes a small fraction of active lithium through side reactions like solid‑electrolyte interphase growth, while elevated temperatures accelerate parasitic processes. Depth of discharge also plays a critical role: deep cycles induce more structural stress in electrode materials than shallow ones. The calculator employs a semi-empirical model that captures these effects using simplified terms. The remaining capacity $\mathrm{C\_r}$ relative to the initial capacity $\mathrm{C\_0}$ is approximated as:

$\frac{\mathrm{C\_r}}{\mathrm{C\_0}}=1-a\sqrt{N}-bA-c\frac{T-25}{10}A-d\frac{D}{100}N$

where $N$ is the number of cycles, $A$ the age in years, $T$ the average temperature in °C, and $D$ the depth of discharge percentage. Coefficients $a=0.0008$, $b=0.01$, $c=0.002$, and $d=0.00002$ are heuristic constants chosen to yield realistic degradation rates.

Risk Assessment

To communicate suitability for second‑life applications, the calculator computes a logistic risk score representing the probability that the battery will fall below 80% capacity within the next three years assuming similar usage. Using the projected remaining capacity and a simple linear degradation continuation, the risk is $\mathrm{Risk}=100\backslash times\sigma \left(\frac{0.8\mathrm{C\_0}-\mathrm{C\_r}}{0.05\mathrm{C\_0}}\right)$. Values under 25% imply the battery is a strong candidate for reuse, while scores above 75% suggest it may soon require recycling.

Guidance Table

Risk %	Second-Life Viability
0‑25	Excellent
26‑50	Good
51‑75	Marginal
76‑100	Poor

Example

Consider an electric vehicle pack that began with 60 kWh of capacity. After five years and 1200 cycles at 90% depth of discharge in a warm climate averaging 30 °C, the calculator estimates a remaining capacity near 44 kWh—about 73% of its original value. The risk metric indicates a high probability of dropping below 80% within three years, meaning the pack may not be ideal for second‑life deployment unless refurbishment or derating strategies are applied.

Practical Considerations

Repurposing batteries requires more than just assessing remaining capacity. Cells must be reconfigured into new modules with appropriate balancing systems, and safety concerns such as thermal runaway must be addressed. Variability between cells within a pack can be significant, necessitating sorting and grading before assembly into second‑life systems. Nevertheless, capacity estimation is a crucial first step, informing economic models and lifecycle analyses. This calculator offers a high‑level perspective to guide further testing and refurbishment decisions.

Limitations

The degradation model used is intentionally simple and should not substitute for detailed diagnostic testing. Real cells exhibit complex behaviors influenced by chemistry, manufacturer quality, fast charging practices, and extreme temperatures. Battery management systems may also limit usable capacity to preserve lifespan, meaning the apparent fade may differ from the actual electrochemical state. Users should supplement the calculator’s output with impedance measurements, cell balancing data, and, where possible, manufacturer‑supplied degradation curves.

Conclusion

The Battery Second-Life Capacity Calculator empowers engineers, hobbyists, and policymakers to quickly gauge the remaining energy storage potential of used lithium‑ion packs. By translating a handful of parameters into a capacity estimate and risk score, the tool supports decisions about reuse versus recycling, helping maximize the environmental and economic benefits of electrification.