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

TL;DR

This study evaluates the trade-off between increased EV battery throughput and the days lost due to calendar aging in V2G services, emphasizing the importance of degradation mechanisms and environmental factors.

Contribution

It introduces a novel throughput versus days lost (TvD) metric and demonstrates how calendar aging impacts V2G viability across different battery chemistries and conditions.

Findings

TvD correlates strongly with lithium inventory loss due to calendar aging.

Battery chemistry and parking temperature are key factors influencing V2G benefits.

Decision to V2G can be based on estimating capacity loss from calendar aging.

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.