Repurposing Electric Vehicle Batteries in a Mixed Array for Grid Storage

Abstract

In the face of the global transition away from fossil fuels, the electricity grid faces many challenges. The growth of renewable energy requires investment in new methods of matching supply and demand with intermittent resources. Energy storage has become a leading technology to meet this challenge. At the same time, electric vehicles are taking to the world’s roads, with 2 million plug-in vehicles in cumulative sales this year. Once their automotive life is completed, the batteries may still be useful for other purposes. Used electric vehicle batteries may be re-purposed for grid electricity storage. Batteries from different manufacturers and use history may be aggregated to optimally draw on the locally available supply of used batteries. A mixed battery array concept is created for a new implementation, along with a list of priority research topics. Five EV batteries are tested according to PNNL suggested protocols, to determine their relative performance. It is found that EV batteries can provide grid services including peak shaving and frequency regulation. In deepdischarge constant-power cycling, energy capacities were within 10% of nominal rated values, with DC energy efficiency between 95-98%, at a 4 hour discharge rate. When increased to a 0.5 hour rate, energy capacity reduced to 50-70% of nominal, and energy efficiency reduced to 85- 95%. When providing frequency regulation services, all batteries reached an apparent limit near a power bid factor equivalent to a 0.5 hour rate. Cooling performance was best with parallel liquid cooling, then parallel forced-air cooling, then series liquid cooling, then passive cooling. Liquid cooling vs. air cooling was not a strong indicator of cooling ability, rather series vs. parallel configuration was the dominant factor. The conclusion drawn from testing is that second-life batteries are technically viable for re-purposing. A performance ranking was created to assist in selecting batteries to provide grid services.