Method of maintaining health of a flow battery

1. A method of maintaining health of a flow battery, the method comprising:

(a) determining an average oxidation state of a common electrochemically active elemental specie in first and second fluid electrolytes on, respectively, a positive side and a negative side of an electrochemical cell of a flow battery; and

(b) adjusting the average oxidation state in response to the average oxidation state deviating from a predefined average oxidation state value to be either higher or lower than the predefined average oxidation state value in response to an expectation that the average oxidation state will creep toward the predefined average oxidation state value.

2. The method as recited in claim 1 , wherein the common electrochemically active elemental specie is selected from the group consisting of vanadium, iron, and chromium.

3. The method as recited in claim 1 , wherein said step (a) includes directly determining the average oxidation state from concentrations of different valence states of the common electrochemically active elemental specie in first and second fluid electrolytes.

4. The method as recited in claim 3 , wherein said step (a) includes determining the average oxidation state as a function of molar concentrations of the different valence states divided by a total molar amount of the common electrochemically active elemental specie.

5. The method as recited in claim 1 , wherein said step (b) includes adding a reducing agent to the second fluid electrolyte on the positive side to reduce a valence state of the common electrochemically active elemental specie in the second fluid electrolyte.

6. The method as recited in claim 5 , wherein the reducing agent includes an acid.

7. The method as recited in claim 5 , wherein the reducing agent includes oxalic acid.

8. The method as recited in claim 5 , wherein the reducing agent includes formic acid.

9. The method as recited in claim 5 , wherein the reducing agent includes an alcohol.

10. The method as recited in claim 1 , wherein said step (b) includes adding an oxidizing agent to the first fluid electrolyte on the negative side to increase a valence state of the common electrochemically active elemental specie in the first fluid electrolyte.

11. The method as recited in claim 10 , wherein the oxidizing agent includes oxygen gas.

12. The method as recited in claim 10 , wherein the oxidizing agent includes air.

13. The method as recited in claim 10 , wherein the oxidizing agent includes hydrogen peroxide.

14. The method as recited in claim 1 , wherein the predefined average oxidation state value is an average oxidation state range.

15. The method as recited in claim 1 , wherein said step (a) includes collecting measurements representing concentrations of different valence states of the common electrochemically active elemental specie in first and second fluid electrolytes.

16. The method as recited in claim 15 , wherein the measurements include concentration measurements.

17. The method as recited in claim 15 , wherein the measurements are selected from the group consisting of optical measurements, conductivity measurements, density measurements, viscosity measurements and combinations thereof.

18. The method as recited in claim 1 , further comprising reducing a charge capacity of the flow battery in response to the average oxidation state deviating from the predefined average oxidation state value.

19. The method as recited in claim 1 , wherein the flow battery comprises:

the electrochemical cell including a first electrode, a second electrode spaced apart from the first electrode and an electrolyte separator layer arranged between the first electrode and the second electrode, and

a supply/storage system external of the electrochemical cell, the supply/storage system including first and second vessels fluidly connected with the electrochemical cell.