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.