**1. Introduction**

The Hawai'i Clean Energy Initiative endeavors to gain total oil independence by 2045 [1] for both electricity generation and ground transportation. On the grid side, the energy portfolio will likely include many intermittent resources such as wind and solar and will thus require significant energy storage. The electrification of ground transportation could kill two birds with one stone by providing oil-free vehicles and offer energy storage when the vehicles are not in use. The latter is referred to as vehicle-to-grid (V2G) and grid-to-vehicle (G2V).

With incentives in place [2,3], the number of electric vehicles (EVs) in Hawai'i and the rest of the world continues to rise and will collectively constitute a significant distributed energy storage reservoir for the grid. EV batteries could provide ancillary grid services such as operating reserves, power curtailment, frequency regulation, and voltage smoothing by allowing the network to give (G2V) and take (V2G) energy when necessary [4,5]. The benefits and drawbacks to both the vehicle owner and the energy provider of these strategies have been well-documented [6–18], and the main obstacle was identified to be the additional usage on the cells [19–21], among other challenges [22]. Few experimental studies [23–28] attempted to account for the change in battery degradation resulting from the implementation of these strategies. To accurately account for the change in usage, the path dependence of degradation needs to be considered in the estimation [29]. Each of these ancillary grid services can affect degradation differently, and certain conditions can lead to accelerated capacity loss [30–32]. This accelerated capacity loss, sometimes termed "rollover failure" [33], is a significant safety concern to the battery industry. However, solutions do exist, and it was shown in a previous work [31] that, although this second stage of degradation cannot be predicted from capacity nor the resistance evolution, it might be predicted from the investigation of the voltage response using electrochemical voltage spectroscopies [29,34,35].

This work is a follow-up of our previous studies [25,26,36,37], in which we purchased a batch of commercial cells to test the impact of different aspects of EV battery usage. Part 1 was devoted to

the definition of the cell-to-cell variations of the full batch and the emulation of the electrochemical behavior [36]. Part 2 focused on the capacity- and resistance-based analyses of cycle- and calendar-aging experiments to assess the impact of bidirectional charging durability [25]. In Part 3, [26], the cycle and calendar-aging degradation mechanisms were investigated using an incremental capacity analysis and the features of interest (FOI) approach [38]. The analysis was used to quantify the different degradation modes, determine the degradation path dependency, and challenge the Part 2 [25] capacity- and resistance-based forecast. Finally, Part 4 was devoted to the study of the impact of different driving cycles on the degradation mechanisms [37].

The next step of this EV battery degradation research, detailed here, is quantifying the consequences of a distinct grid interaction: frequency response. This investigation elucidates the impact of frequency regulation under several conditions, including a new approach of modulating the charge so that the vehicle can perform grid applications without any additional battery usage.
