(Invited) Statistical Distribution and Feasibility for Re-Use of A123 LiFePO4 Cells from a Hybrid-Bus Battery Pack

Second-use applications for batteries can play an important role in reducing the environmental impacts and life-cycle costs of electrochemical energy storage. The economic feasibility of re-using a battery pack is strongly influenced by the state-of-health (SOH) of its modules and cells after primary use. Thus, there is a strong incentive to optimize procedures for diagnosing SOH based on parameters such as calendar life, cycle life, internal resistance, and capacity fade. In this paper, we analyze the statistical distribution of the SOH cells extracted from a retired BAE-Systems ESS-A123 hybrid bus battery pack. The pack consists of 16 modules in series, each containing 96 lithium-iron-phosphate 26650 cells in a 12s8p configuration. Most of the modules use ANR26650M1-A (M1-A) cells. Two modules use M1-B cells—a newer version of the same battery—indicating that these two modules were installed as replacements. The M1-A and M1-B cells have nominal capacities of 2.3 and 2.5 Ah, respectively. We randomly sampled one cell from each of the 192 sub-modules that result from the 8p design and performed a comprehensive SOH assessment. We will summarize the data by presenting internal resistance, discharge capacity, Warburg diffusion coefficient, and charge-transfer resistance as a function of cell location and type. Results show that the M1-A cells display widely varying SOH, while the M1-B cells are in much better condition. The wide range in SOH parameters suggest re-use of all parts of the pack may not be economically reasonable. We will also present an analysis of the duty profile of the bus battery. We will then discuss the implications of our findings for the economic feasibility of re-using the cells and modules, in light of available data and modeling techniques.