Grace Capacity Utilization of NMC and LFP Battery Cells

Understanding the Role of Grace Capacity in EV Batteries

Grace capacity refers to the additional battery capacity beyond the rated capacity specified by manufacturers. It serves as a safety buffer against natural battery degradation and protects against inefficient charging habits. Battery and EV manufacturers prioritize maintaining a consistent user experience by ensuring that the available capacity remains stable over time. To achieve this, manufacturers gradually reduce grace capacity as battery degradation progresses, thereby minimizing fluctuations in the usable capacity for users.

Methodology and Simulation Approach

This study analyzes the impact of grace capacity on the actual performance and lifespan of NMC (Nickel-Manganese-Cobalt) and LFP (Lithium Iron Phosphate) battery cells. Commercially available NMC 18650 and LFP battery cells were examined using charge-discharge cycle data under varying Depth of Discharge (DoD) and End of Charge (EoC) conditions. The study hypothesizes that at a specific Stage of Health (SoH), defining DoD and EoC results in a shared charge-discharge profile with a fixed ratio. Based on this assumption, simulations were conducted. Furthermore, Python-based simulations were performed using cycle experiment data to analyze trends in grace capacity variations while maintaining different capacity levels. Machine learning techniques were employed to develop linear and exponential regression models, enabling a quantitative evaluation of how grace capacity adjustments influence battery performance.

Key Findings and Implications

The simulation results revealed that inducing various grace capacity adjustments led to different total energy utilization trends until the end of life for NMC and LFP batteries. These findings highlight the distinct degradation mechanisms of each battery chemistry and underscore the impact of grace capacity optimization strategies on EV battery management.

This study was presented at the 2024 Korean Society of Automotive Engineers (KSAE) Fall Conference poster session, where discussions centered on the effects of various grace capacity strategies on EV battery performance and longevity.

All photos are taken and edited by me