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.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
 Inventions conceived after the filing of the priority application (U.S. patent application Ser. No. 12/498,103, filed on Jul. 6, 2009) that are included in this continuation-in-part patent application were made with Government support under DE-OE0000225 "Recovery Act--Flow Battery Solution for Smart Grid Renewable Energy Applications" awarded by the US Department of Energy (DOE). The Government has certain rights in such inventions. However, the Government does not have rights in U.S. Pat. No. 7,820,321 which was conceived and filed without Government support, nor in the direct continuation and divisional applications thereof.