Battery management reduces this effect by keeping operating conditions stable and minimizing stress on individual cells. The result is more consistent performance, better use of available capacity, and slower long-term degradation. Understanding the degradation behavior of lithium-ion batteries under realistic application conditions is critical for the design and operation of Battery Energy Storage Systems (BESS). This research presents a modular, cell-level simulation framework that integrates electrical, thermal, and aging. Renewable energy systems rely on batteries to store power when generation does not match demand. These systems often charge and discharge daily, sometimes multiple times per day. In this Review, we describe BESTs being developed for grid-scale energy storage, including.