Battery lifetime is an academic concept and research field concerned with the duration over which a battery maintains acceptable performance levels before reaching an end-of-life criterion. This field investigates the complex physical and chemical degradation mechanisms that limit a battery's capacity, power capability, and overall service life under varying operational and environmental conditions. Key characteristics explored include cycle life, calendar life, and the definition of end-of-life metrics, with significant importance for the reliability, economic feasibility, and sustainable deployment of battery technologies in diverse applications.