Repurposed electric vehicle battery performance in second-life electricity grid frequency regulation service

Abstract

Repurposing of electric vehicle (EV) batteries for a “second life” in electricity grid applications is a potential dual solution for extending the value of these high-quality batteries beyond EV service while reducing the capital cost of grid-scale energy storage. Given the diversity among EV batteries in terms of size, shape, chemistry, performance, and thermal management, it is expected that the practicality and performance of different brands and models will vary in a second-life application. To develop this energy storage technology, common EV battery types must be characterized and ranked according to their performance in different grid services. This experimental study evaluates six different EV batteries with different EV use histories for performance in electricity grid frequency regulation (FR) service using a novel methodology based on the protocol developed by Sandia National Laboratories. For each battery, the applied FR power signal is scaled to greater magnitudes in sequential tests to evaluate trends in energy efficiency and thermal performance. It is found that dc energy efficiency of EV batteries in FR service is primarily influenced by positive active material type and ranges from 92% (NCA) to 99% (NMC) for mid-range FR power magnitudes. Batteries featuring active air or active liquid thermal management are found to maintain safe operating temperatures at substantially higher FR power magnitudes compared to passive thermal management. The results are used to rank the tested EV batteries for commercial performance in second-life FR service, with practical consideration of the trade-offs between metrics in relation to different potential project constraints.