Extra Throughput versus Days Lost in load-shifting V2G services: Influence of dominant degradation mechanism

TL;DR

The paper tackles the unclear impact of vehicle-to-grid (V2G) load-shifting on EV battery life by developing physics-based digital-twins for three cell families with distinct dominant degradation mechanisms. It introduces TvD, a non-dimensional metric that compares throughput gained from V2G to days lost due to aging, and demonstrates a strong link between TvD and the calendar-aging fraction $rac{ ext{LLI}_{ ext{Cal}}}{ ext{LLI}}$. By tuning a comprehensive intra- and inter-cycle degradation model to calendar and cycling aging data and simulating V2G and noV2G scenarios, the authors show that calendar-dominated cells can reap substantial V2G benefits, while cycle-dominated cells may incur higher time-based degradation with limited throughput gains. The results offer a principled way to assess V2G viability, inform warranty policies, and motivate deployment strategies that account for cell chemistry, temperature, SOC, and duty-cycle timing. The work provides a transferable, open-source digital-twin framework to explore V2G opportunities across future battery technologies and operating contexts.

Abstract

Electric vehicle (EV) batteries are often underutilized. Vehicle-to-grid (V2G) services can tap into this unused potential, but increased battery usage may lead to more degradation and shorter battery life. This paper substantiates the advantages of providing load-shifting V2G services when the battery is aging, primarily due to calendar aging mechanisms (active degradation mechanisms while the battery is not used). After parameterizing a physics-based digital-twin for three different dominant degradation patterns within the same chemistry (NMC), we introduce a novel metric for evaluating the benefit and associated harm of V2G services: \textit{throughput gained versus days lost (TvD)} and show its strong relationship to the ratio of loss of lithium inventory (LLI) due to calendar aging to the total LLI ($\text{LLI}_\text{Cal}/\text{LLI}$). Our results that focus systematically on degradation mechanisms via lifetime simulation of digital-twins significantly expand prior work that was primarily concentrating on quantifying and reducing the degradation of specific cells by probing their usage and charging patterns. Examining various cell chemistries and conditions enables us to take a broader view and determine whether a particular battery pack is appropriate for load-shifting (V2G) services. Our research demonstrates that the decision "to V2G or not to V2G" can be made by merely estimating the portion of capacity deterioration caused by calendar aging. Specifically, TvD is primarily influenced by the chemistry of cells and the environmental temperature where the car is parked, while the usage intensity and charging patterns of EVs play a lesser role.

Figures (8)

• Figure 1: Scenarios in this study: (a) Daily duty cycles (24 hr) with and without V2G are considered, including one hour of driving to work and an hour back home. The V2G duty cycle includes 2 hours of discharge to the grid divided equally between morning and afternoon. The average SOC is equal to 0.79 for both duty cycles. (b) The current profile (as C-rate) of the drive cycle includes Federal test procedures with an accumulated driving distance of 34.1 miles each hour. The C-rate distribution of each drive cycle is also presented.
• Figure 2: The resulting SOCs in the first cycle for the noV2G, V2G, early-charge V2G, and late-charge V2G scenarios. The average state of charge $(\text{SOC}_\text{ave})$ is shown with a solid black line for each scenario. The noV2G and V2G cases have the same average SOC; thus, we call this case moderate V2G. The early-charging V2G case rests longer at higher voltage and has a larger average SOC. Late-charging V2G rests at a lower voltage and has a smaller average SOC.
• Figure 3: Summary of degradation mechanism equations based on SPM. Cathode dissolution and SEI growth are the only mechanisms that are active during calendar aging. All the mechanisms are present during the cycling of the cell.
• Figure 4: Summary of fitting process. First, the parameters of SEI and cathode dissolution models are found. These parameters, along with cycle test data, are used to find the parameters of Li-plating and mechanical degradation models. An adaptive simulation is used to reduce the simulation time needed for every lifetime iteration to be performed.
• Figure 5: Comparison of the experimental data and fitted model simulations for the three cells. In (a) capacity loss, LAM in the negative and positive electrodes, and (b) voltage behavior of the fresh and aged cells during cycling are shown. For NMC111, the voltage shown is C/5 charge, C/5 discharge at 50% DOD. For NMC622-25C, the cell is charging and discharging at C/2 at 50% DOD. NMC622-45C has cycled at 1C charge and discharge rate at 100%.