Field testing of repurposed electric vehicle batteries for price-driven grid balancing

Abstract As electric cars become more widespread, the disposal and recycling of used batteries will become an important challenge. Typically, vehicle batteries are replaced if their capacity drops to 70–80% of initial capacity. However, they may still be useful for stationary applications. In this paper, results from a field test of a molten salt high-temperature electric vehicle battery repurposed as stationary storage for grid balancing are presented. In a previous study, we have shown that a mixed integer linear programming control strategy driven by a spot-market price for electricity is best suited for an implementation on hardware with limited computational resources. A 14-day experiment resulted in a round-trip energy efficiency (converter-battery-converter) of about 74.4%. The earnings per battery capacity per day achieved in this period amounted to 7.38 €/MWh. An error analysis of the model underlying the optimization showed a root mean square error of 7.6% between the estimated and measured state of charge. The field test implementation shows a substantial economic deviation of 37.3% between theoretical and physical potential of grid-balancing measures due to model inaccuracies and technical characteristics, thereby demonstrating the urgent need for field tests of repurposed electric vehicle batteries for stationary applications.