CellSage is a robust tool for battery health modeling, simulation, and analysis, delivering precision insights into lithium-ion battery performance under real-world conditions. Results from a decade of research supported by the Idaho National Laboratory, this platform adopts hybrid modeling techniques that combine data-driven and physics-based approaches. Users get a comprehensive suite for analyzing aspects like capacity fade, thermal management, and operational parameters impacting battery life. CellSage is essential for sectors needing reliable battery management, helping reduce upfront R&D costs and accelerate development timelines. It further supports optimal battery management, enhancing safety and efficiency. Designed for academic and industrial researchers, CellSage facilitates dynamic interaction and integration with Battery Management Systems (BMS) and Integrated Vehicle Health Monitoring systems. It aids in ensuring precise state-of-health assessments and RUL predictions, thus supporting Condition-Based Maintenance. This capability not only improves operational continuity but also extends the battery lifespan, enabling forward-thinking battery technology.