NOVELTY - The system has flow battery cells, which are arranged along reactant flow path which has first and second ends. The outlet of first flow battery cell is coupled to the inlet of second flow battery cell. The first flow battery cell has first structural configuration and material configuration, according to the first position along the reactant flow path. The second flow battery cell has second structural configuration and a material configuration, according to second position along the reactant flow path.    USE - Flow battery energy storage system e.g., redox flow battery energy storage system. Uses include but are not limited to nuclear and coal-fired power plants, power grid application, industrial facility, gravity feed system, and electric vehicle (EV) or plug-in hybrid electric vehicle (PHEV) charging station.    ADVANTAGE - Enhances overall efficiency and battery performance. Improves the roundtrip direct current (DC) efficiency of redox flow battery stack assembly and eliminates the need to re-circulate the reactants. Avoids economic penalties for failing to supply contracted power levels during times no wind. Enables the output power to reliably follow the DC load, without power spikes or power drops. Eliminates the need for pumps, thus reducing the cost and maintenance requirements. Prevents mixing of charged and discharged electrolyte during both charging and discharging cycles, which results to lower system cost, more constant power output, and higher DC efficiency. Provides a system design that yields an energy storage system that is readily transported and set-up at the destination, and that facilitates control over electrolyte flows and full or partial draining of the stacks when the battery system is idle for short or extended periods of time.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic block diagram of a large stack redox battery system.    Redox flow battery stack assembly (10)    Membrane separator (12)    Electrical switch (44)    Power source (45)    Load (46)