A large stack redox flow battery system provides a solution to the energy storage challenge of many types of renewable energy systems. Independent reaction cells arranged in a cascade configuration are configured according to state of charge conditions expected in each cell. The large stack redox flow battery system can support multi-megawatt implementations suitable for use with power grid applications. Thermal integration with energy generating systems, such as fuel cell, wind and solar systems, further maximize total energy efficiency. The redox flow battery system can also be scaled down to smaller applications, such as a gravity feed system suitable for small and remote site applications.