An improved redox battery energy storage system is disclosed for reducing oxygen gas levels and separating oxygen gas from hydrogen gas, thus reducing the likelihood of flammable gas explosions. The system includes at least one cell, which includes a positive compartment having positive solution, a negative compartment having negative solution, and a membrane separating the positive and negative compartments. A positive reservoir is in fluid communication with the cell's positive compartment, the positive reservoir defining a positive vent space for positive gas, which includes oxygen. A negative reservoir is in fluid communication with the cell's negative compartment, the negative reservoir defining a negative vent space. A return line is in fluid communication with the negative compartment and the negative reservoir to return the negative solution from the cell to the negative reservoir through the negative vent space. A positive gas draw line is in fluid communication with the positive vent space and coupled to the return line at a connection, causing positive gas to be siphoned from the positive vent space into the negative vent space, where the negative solution of the negative reservoir scavenges the oxygen. A balancing line is in fluid communication with the positive and the negative vent spaces to recirculate oxygen-depleted positive gas to the positive vent space.