Battery second use-putting used plug-in electric vehicle (PEV) batteries into secondary service following their automotive tenure-has been proposed as a means to decrease the cost of PEVs while providing low cost energy storage to other fields (e.g., electric utility markets). To understand the value of used automotive batteries, however, we must first answer several key questions related to battery degradation, including: How long will PEV batteries last in automotive service? How healthy will PEV batteries be when they leave automotive service? How long will retired PEV batteries last in second-use service? How well can we best predict the second-use lifetime of a used automotive battery? Under the support of the U.S. Department of Energy's Vehicle Technologies Office, the National Renewable Energy Laboratory has developed a methodology and the requisite tools to answer these questions, including the Battery Lifetime Simulation Tool (BLAST). Herein we introduce these methods and tools and demonstrate their application. Under our assumed second use duty cycle of daily peak shaving, we have found that repurposed automotive batteries can last ten years or more in second use service when managed properly. In this role, capacity fade from automotive use has a much larger impact on second use value than resistance growth. Where capacity loss is driven by calendar effects more than cycling effects, average battery temperature during automotive service, which is often driven by climate, is found to be the single factor with the largest effect on remaining value. Installing hardware and software capabilities onboard the vehicle that can both estimate remaining battery capacity from in-situ measurements, as well as track average battery temperature over time, will thereby facilitate the second use of automotive batteries.