As electric vehicle (EV) adoption accelerates globally, a growing volume of lithium-ion batteries are reaching end-of-life in their primary automotive application—despite retaining 60 to 80% of their original capacity. This presents a significant opportunity to extend battery utility through second-life applications such as stationary energy storage, microgrid support, and commercial backup systems. This paper analyzes the strategies for maximizing the residual value of second-life EV batteries through repurposing and resale, while also addressing the challenges associated with performance optimization and standardization of testing and certification procedures. The study evaluates the techno-economic viability of second-life batteries compared to new systems, emphasizing cost savings, environmental impact, and emerging market demand. Techniques for enhancing second-life performance are examined, including advanced state-of-health (SoH) diagnostics, machine learning models for usage profiling, and improved thermal and battery management systems (BMS). These methods are critical for ensuring safe, reliable, and economically viable reuse of retired EV batteries. A core challenge in second-life battery deployment is the lack of harmonized standards for testing, grading, and certifying used battery packs and modules. The paper reviews existing standards such as UL 1974, SAE J2997/J2998, IEC 62933-2-1 and draft standards such as IEEE P2993, identifying key limitations in scalability, interoperability, and regional regulatory alignment. A framework proposed is for standardization, including unified diagnostic protocols, digital battery passports for traceability, and scalable certification platforms. Through an integrated analysis, the paper underscores the need for collaborative efforts among manufacturers, regulators, and technology providers to unlock the full circular economy potential of EV batteries. By aligning repurposed strategies with robust testing and certification frameworks, second-life EV batteries can deliver both economic returns and significant environmental benefits, advancing global energy sustainability goals.