*Article* **Cell Replacement Strategies for Lithium Ion Battery Packs**

#### **Nenad G. Nenadic \*,†, Thomas A. Trabold † and Michael G. Thurston †**

Rochester Institute of Technology, Rochester, NY 14623, USA; tatasp@rit.edu (T.A.T.); mgtasp@rit.edu (M.G.T.) **\***

 Correspondence: nxnasp@rit.edu; Tel.: +1-585-662-8250

† These authors contributed equally to this work.

Received: 13 May 2020; Accepted: 17 July 2020; Published: 23 July 2020

**Abstract:** The economic value of high-capacity battery systems, being used in a wide variety of automotive and energy storage applications, is strongly affected by the duration of their service lifetime. Because many battery systems now feature a very large number of individual cells, it is necessary to understand how cell-to-cell interactions can affect durability, and how to best replace poorly performing cells to extend the lifetime of the entire battery pack. This paper first examines the baseline results of aging individual cells, then aging of cells in a representative 3S3P battery pack, and compares them to the results of repaired packs. The baseline results indicate nearly the same rate of capacity fade for single cells and those aged in a pack; however, the capacity variation due to a few degrees changes in room temperature (±3 ◦C) is significant (±1.5% of capacity of new cell) compared to the percent change of capacity over the battery life cycle in primary applications (20–30%). The cell replacement strategies investigation considers two scenarios: early life failure, where one cell in a pack fails prematurely, and building a pack from used cells for less demanding applications. Early life failure replacement found that, despite mismatches in impedance and capacity, a new cell can perform adequately within a pack of moderately aged cells. The second scenario for reuse of lithium ion battery packs examines the problem of assembling a pack for less-demanding applications from a set of aged cells, which exhibit more variation in capacity and impedance than their new counterparts. The cells used in the aging comparison part of the study were deeply discharged, recovered, assembled in a new pack, and cycled. We discuss the criteria for selecting the aged cells for building a secondary pack and compare the performance and coulombic efficiency of the secondary pack to the pack built from new cells and the repaired pack. The pack that employed aged cells performed well, but its efficiency was reduced.

**Keywords:** capacity fade; secondary applications; end-of-life; cell balancing; temperature effects
