Results of a study by a team at the Hawaii Natural Energy Institute, SOEST, University of Hawaii at Manoa, suggest that the additional cycling to discharge vehicle batteries to the power grid in a vehicle-to-grid (V2G) scenario, even at constant power, is detrimental to EV battery cell performance. This additional use of the battery packs could shorten the lifetime for vehicle use to less than five years, the researchers said in a paper published in the Journal of Power Sources.
By contrast, the researchers found that delaying the grid-to-vehicle (G2V) charge in order to reduce the impact on the power grid had a negligible impact on the cells at room temperature, but could be significant in warmer climates.
Given electric vehicles (EVs) projected penetration into the market, their batteries have the potential to provide significant energy storage for the electric power grid in the future by allowing the grid to give and take energy from the batteries when needed. This bidirectional charging is known as Grid-to-vehicle (G2V) and Vehicle-to-grid (V2G) respectively. … The potential of using EV battery capacity for grid storage has been the subject of much discussion in recent years but few studies have tested grid impact on EV battery degradation. … Most battery-related work published so far focuses on modeling. Although some studies stand out, the models unfortunately do not represent realistic battery degradation.
Understanding the real impact of bidirectional charging on batteries is essential to weigh their viability as a grid support service. Indeed, V2G will induce more usage of the cells by discharging to the power grid and having to recharge the battery again for transportation. Controlled G2V on the other hand will allow the cells to rest at a different state of charge (SOC) compared with charging immediately upon arrival, and therefore might induce a different calendar aging. Since battery degradation is path dependent, any changes in usage can lead to different degradation mechanisms and drastically influence the reliability of the batteries.—Dubarry et al.
The Hawaii team performed laboratory testing on commercial Li-ion cells to investigate the impact of bidirectional charging on Panasonic 18650 NCA batteries. The researchers investigated the effects of V2G/G2V combined with different charging schedules (1 or 2 charges a day, immediate or delayed charging) and different charging currents (level 2 or fast charging). Further, the effect of calendar aging at different temperatures was also investigated in a second set of experiments.
The team concluded that a V2G step twice a day increased battery capacity loss by 75% and the resistance by 10%. This step once a day accelerated the capacity loss by 33% and the resistance increase by 5%. Forecasts based on the measurement results indicated that that V2G implementation would decrease the lifetime of the battery packs to under 5 years.
The team also found that calendar aging influenced the cells little enough that it was beneficial to charge the cells twice a day instead of once. Charging twice per day resulted in 5% less capacity loss and similar resistance increase compared to once per day.
Matthieu Dubarry, Arnaud Devie, Katherine McKenzie (2017) “Durability and reliability of electric vehicle batteries under electric utility grid operations: Bidirectional charging impact analysis,” Journal of Power Sources, Volume 358, Pages 39-49, doi: 10.1016/j.jpowsour.2017.05.015