The energy storage system (ESS) safety issue caused by battery thermal runaway is becoming severe, and its solution for perfect fire suppression method has not yet been optimized. Herein, we propose the concept of a Water-in-Battery (WiB) system, which utilizes water for functional elements in an existing battery to manage the generated heat and remove the fire-causing elements in the battery. Under general conditions (at high C-rates and high temperatures), the system acts as a coolant and extends the battery life. Under abuse conditions such as overcharging or overheating, the water in the WiB delays the thermal runaway of the battery. In addition, the system blocks fire by eliminating oxygen and decreasing the temperature of the battery. Moreover, this system can be simply implemented in an existing ESS by a low-cost and eco-friendly method.