**4. Conclusions**

This study examined fundamental properties of aging lithium ion battery cells. We described the testing methodology and established that the cells in a carefully designed pack aged at the same rate as when they are individually aged. The careful design consisted of matching overall capacities of serial connections to minimize charge exchanges between strings and the associated dissipation, and also to match impedances within a serial connection to minimize equalization with its associated losses. We then considered two common scenarios in cell replacements: replacing a prematurely failed cell and building a new pack from cells of the damaged packs for less demanding applications. Less demanding applications considered here are those that can tolerate capacity of less than 70%. Stationary applications, e.g., microgrid storage, where the overall pack weight is not a limiting factor, are considered good examples of less demanding applications that can be source their cells and modules from high-demanding applications (e.g., transportation).

The first scenario is important for maintenance of existing packs and modules, especially large packs. We found that, at least for demanding applications where the pack operates until its cell drop to about 80% of the original capacity, a new cell coexisted well within a pack of aged cells. Therefore, within these applications, there may not be a need for a potentially expensive process of pre-aging cells.

The second scenario addressed the problem of repurposing of used cells in less demanding applications. To examine a severe case of a pack failure, we deeply discharged two packs and assembled a pack from the surviving cells. The deep discharge event was so severe that 6 of the original 18 cells were permanently damaged. Two approaches to rebuilding the packs were considered, but only matching the cells within a string worked robustly in our pack. After the pack was rebuilt in this fashion, more than forty cycles were successfully completed. Comparing the modes of the distribution, it was found that the overall pack efficiency of the rebuilt pack was about 8% lower than that of pack comprised of new cells. However, the heating of individual cells within the three packs was compared and no significant differences were found. We finally showed that increased losses due to larger mismatches can be partially compensated by employing active instead of passive cell balancing. The balancing scheme may be particularly critical for secondary applications where cells cannot be matched as well as in the primary applications, and where the initial mismatches are expected to grow further over time.

Lessons learned from this study will be employed in our future work of assessing opportunities for repurposing larger packs, where secondary applications may have specific use profiles distinctly different from the primary applications. For example, this information could be used in considering reuse of vehicle batteries in grid applications, such as peak shaving and smoothing of solar-generated power vs. their reuse in much smaller systems such as power tools, or toys. The future work may also consider other criteria, including economic and environmental performance.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2313-0105/6/3/39/s1, Figure S1: High-level view of the research program; Figure S2: Initial characterization and single-cell testing; Figure S3: Histogram of cell weights; Figure S4: Characteristic waveforms of a typical cycle; Figure S5:Impedance spectroscopy. (a) Average spectra. (b) Repeatability and potentiostat comparison; Figure S6: Schematic diagram of the 3S3P battery pack; Figure S7: (a) LabVIEW-controlled battery pack test stand. (b) Enlarged view of the pack; Figure S8:Voltage and current waveforms for a cell during pack cycling; Figure S9: Current distribution of three strings over four cycles; Figure S10:Voltage distribution of three cells within string 1 over four cycles; Figure S11: Cell balancing for one of the strings. Pack design employed 106 Astro "Blinky" from AstroFlight Inc. as the commercial cell balancer; Figure S12: Voltage distribution of three cells within string 1 over four cycles for active cell balancing; Figure S13:(a) Representative capacity of individual cells diverging in time (18650, lithium cobaltoxide chemistry). (b) Scatter plot of capacity fade vs. percent recovery; Figure S14: Capacity fade of eight lithium cobalt cells; Figure S15: Thermal image of the pack; Figure S16: Comparison of heating of lithium ion batteries; Table S1.Effect of *Ta* on *Ted*; Algorithm S1: Charge cycle.

**Author Contributions:** Conceptualization, N.G.N., T.A.T., and M.G.T.; methodology, N.G.N. and T.A.T.; software, N.G.N.; validation, N.G.N., T.A.T., and M.G.T.; formal analysis, N.G.N. and T.A.T.; investigation, N.G.N., T.A.T., and M.G.T.; data curation, N.G.N.; writing—original draft preparation, N.G.N.; writing—review and editing, N.G.N., T.A.T., and M.G.T.; visualization, N.G.N.; supervision, M.G.T.; project administration, M.G.T.; funding acquisition, M.G.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was made possible by the Office of Naval Research under Award No. N0004-07-1-0823.

**Disclaimer**: Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the Office of Naval Research.

**Acknowledgments:** We gratefully acknowledge the help of our colleagues from Rochester Institute of Technology: Scott Dewey for his assistance in pack assembly and instrument setup, Robert Kosty for his assistance in collecting data and debugging the fixtures, and Joseph Wodenscheck and Art Dee for skillful LabVIEW implementation of the charge cycle and discharge cycle on the battery pack test stand.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
