Vehicle-to-grid (V2G) and grid-to-vehicle (G2V) strategies are often cited as promising approaches to mitigate the intermittency of renewable energy on electric power grids. However, their impact on the vehicle battery degradation has not been investigated in detail.

The aim of this work was to understand the impact of bidirectional charging on commercial Li-ion cells used in electric vehicles today. The influence of V2G and G2V strategies and calendar aging on capacity loss, resistance increase, and rate capability for commercial graphite / LiNi_xCo_1-x-yAl_yO₂batteries were studied.

Results showed that additional cycling to discharge vehicle batteries to the power grid, even at constant power, was detrimental to cell performance. A V2G step once a day was found to accelerate the capacity loss by 33% and the resistance increase by 5%. This step twice a day increased the capacity loss by 75% and the resistance by 10%. In contrast, delaying the charge (G2V) over immediate charging had no significant effect (< 1%) on the capacity retention and a limited impact on the resistance (< 5%) at room temperature. Charging twice per day resulted in 5% less capacity loss and similar resistance increase compared to charging just once daily. In the calendar aging experiments, the capacity loss as a function of time, SOC, and temperature was modeled using a quadratic equation. Of the factors investigated, storage temperature was found to have the biggest impact on capacity loss and rate capability; while, the SOC had the most deleterious effect on the resistance.

Our study concluded that EVs using similar cells should not be left fully charged in warmer climates. Moreover, predictions based on the measurements indicated that V2G implementation would decrease the lifetime of the battery packs below 5 years. The impact of delaying the charge in order to reduce the impact on the power grid, was found to be negligible at room temperature, but could be significant in warmer climates.