This paper investigates the cycle aging behavior of nickel-manganese-cobalt (NMC) lithium ion battery cells under high current rates and narrow window of State-of-Charge (SOC) variation for 48 V mild hybrid electric vehicle (HEV) applications. The charge sustaining aging profiles were synthesized from real-world duty cycles in some of the authors’ previous work [44,45] and used in the laboratory for life cycle aging testing spaced out by periodic cell capacity tests performed to assess cell aging conditions. Furthermore, at each 1C discharged aging characterization stage, the parameters of a physics-based electrochemical cell model were identified using the Bayesian Markov Chain Monte Carlo (MCMC) method. Among the parameters that can be identified with sufficient accuracy from the experimental data, the concentration of lithium-ions at the negative electrode after full charge is found to be strongly correlated with the system-level loss of capacity. These results provide new insights on the aging performance of NMC lithium ion cells in battery management system for 48 V hybrid vehicles.