The determination of the state of health of a battery cell is of critical importance when high discharge rates are necessary (>10C). The US Naval Research Laboratory (NRL) has performed work previously to determine a single frequency using Electrochemical Impedance Spectroscopy (EIS) that can describe the state of health for a single LiFePO4 cell. However, a battery system that can be used in the field requires multiple cells in parallel. Thus, NRL has expanded the previous work to evaluate four cells connected in parallel. The first array was constructed of cells carefully selected for the same equivalent series resistance (ESR) at 1 kHz, the same capacity to within 25 mAH, and nearly identical EIS spectra from 1 kHz – 1 Hz. Four replicant arrays were subsequently constructed of cells with identical ESR and capacity to represent the battery pack construction by a manufacturer. Each array will be pulse discharged at 10C (26 A) using a discharge profile identical to a pulsed power system at NRL. Results from the lifetime testing will be presented, with an electrochemical model and a post-mortem analysis of the battery film surfaces.