A redox flow battery system is provided with independent stack assemblies dedicated for charging and discharging functions. In such a system, characteristics of the charging stack assembly may be configured to provide a high efficiency during a charging reaction, and the discharging stack may be configured to provide a high efficiency during a discharging reaction. In addition to decoupling charging and discharging reactions, redox flow battery stack assemblies are also configured for other variables, such as the degree of power variability of a source or a load. Using a modular approach to building a flow battery system by separating charging functions from discharging functions, and configuring stack assemblies for other variables, provides large-scale energy storage systems with great flexibility for a wide range of applications.