Electrical and Thermal Performance Evaluations for Repurposed Electric Vehicle Batteries in Second Life Electricity Grid Energy Storage

Abstract

The yearly production of lithium-ion batteries for electric vehicles (EVs) is expected to increase sevenfold by 2035 to enable decarbonization of the transportation sector. When an EV battery is removed from EV service, it can usually be repurposed to serve a ‘second life’ in electricity grid energy storage before it is finally recycled to recover key materials. Second life batteries have been the subject of much research and development in recent years, but there are still many challenges that need to be overcome in this new industry. For example, since there are many different EV battery designs with unique performance characteristics, second-life battery researchers and developers must have the means to characterize the electrical and thermal performance of any EV battery in a consistent and comparable way. The adoption of appropriate performance evaluation methods would allow researchers and developers to assess and compare the various strengths and weaknesses of different EV batteries for the purposes of modelling, designing, and operating second-life battery systems.

This thesis presents novel research focused on evaluating the second-life performance of used EV batteries. The objective is to develop suitable electrical and thermal performance evaluation methods, and to use these methods to compare the performance of different EV battery designs. To achieve this objective, five unique EV batteries are acquired and tested at the pack level using a series of constant power tests. The test results are used to compare the five battery packs according to 21 performance metrics, revealing significant performance differences among different EV battery designs. Despite these differences, the test results are shown to vary predictably with the applied power rate based on a set of novel empirical equations, leading to a newly proposed method for standardized second life battery performance characterization. A specialized thermal model is also developed to facilitate real-time monitoring of temperature distributions in second life battery packs. Altogether, the research outcomes contribute powerful new tools and practical insights for the second-life battery industry.