The All-in-One Liquid-Cooled Battery Energy Storage System (BESS) is an integrated, plug-and-play solution that transitions energy storage from engineering-based to product-based models, combining high-performance cells, a battery management system (BMS), a high-performance power conversion system (PCS), a fire protection system, and a liquid cooling system into a single standardized outdoor cabinet. Featureing an AC-DC integrated design, the system uses 314 Ah LiFePO₄ cells in a 1P64S PACK configuration, achieving a high energy density of 158 kWh/m². The PCS and BMS are integrated to reduce line losses, resulting in a system efficiency of over 91%. A specialized flow-guided liquid cooling design is adopted, with real-time temperature monitoring of all cells linked to the BMS, achieving extreme temperature management with a cell temperature difference within 2°C. The system features a multi-level fire protection system that includes PACK-level and cabinet-level perfluorohexanone, supplemented by a water-based fire protection system, and is linked with the BMS and EMS to form a multi-layered, comprehensive safety protection system with high overall safety. The cabinet adopts a single-door, single-opening design with front cable outlet and partitioned battery compartments, minimizing footprint, simplifying installation, and resolving condensation issues. The system is suitable for a wide range of commercial and industrial applications, including peak load shifting, peak-valley balancing, backup power supply, distributed renewable energy integration, and power stability and optimization.