A redox flow battery array comprising: At least two flow battery unit strings (SA, SB) electrically connected in parallel wherein each flow battery unit string (SA, SB) comprises at least one flow battery unit (A1, B1) comprising: a flow battery string comprising at least one flow battery cell (2a, 2b) wherein each cell comprises an anode (3a, 3b), a cathode (4a, 4b), and flow battery unit electrolytes comprising an anolyte (5a, 5b) and a catholyte (6a, 6b); an anolyte tank (7a, 7b) and a catholyte tank (8a, 8b) for storage of the anolyte (5a, 5b) and the catholyte (6a, 6b) of the flow battery unit electrolyte respectively; an anolyte circuit (9a, 9b) comprising a supply line (10a, 10b) and a return line (11a, 11b) connecting the anolyte tank (7a, 7b) to the anodes (3a, 3b) in the flow battery cells (2a, 2b) in the flow battery string; a catholyte circuit (12a, 12b) comprising a supply line (13a, 13b) and a return line (14a, 14b) connecting the catholyte tank (8a, 8b) to the cathodes (4a, 4b) in the flow battery cells (2a, 2b) in the flow battery string; and a state-of-charge measuring device (17a, 17b) capable of measuring the state of charge of the flow battery unit electrolyte in the flow battery unit (A1, B1); a flow regulator in at least one of the anolyte and the catholyte circuits (9a, 9b, 12a, 12b) regulating flow of at least one of the anolyte (5a, 5b) and the catholyte (6a, 6b) therethrough in at least one of the flow battery units (A1, B1) in the at least two flow battery unit strings (SA, SB); a controllable bi-directional power converter electrically connected to each of the at least two flow battery unit strings (SA, SB); and a controller with inputs (20a, 20b) connected to each of the state-of-charge measuring devices (17a, 17b) in the flow battery units (A1, B1) and with an output (21a, 21b) connected to the flow regulator wherein the output of the controller controls the flow regulator so as to regulate the flow of at least one of the anolyte (5a, 5b) and the catholyte (6a, 6b) to the anodes (3a, 3b) and cathodes (4a, 4b) in the flow battery cells (2a, 2b) in the flow battery string in accordance with the states of charge of the flow battery unit electrolytes in the flow battery units (A1, B1) and thereby balance the state-of-charge in the at least two flow battery unit strings (SA, SB). At least two flow battery unit strings (SA, SB) electrically connected in parallel wherein each flow battery unit string (SA, SB) comprises at least one flow battery unit (A1, B1) comprising: a flow battery string comprising at least one flow battery cell (2a, 2b) wherein each cell comprises an anode (3a, 3b), a cathode (4a, 4b), and flow battery unit electrolytes comprising an anolyte (5a, 5b) and a catholyte (6a, 6b); an anolyte tank (7a, 7b) and a catholyte tank (8a, 8b) for storage of the anolyte (5a, 5b) and the catholyte (6a, 6b) of the flow battery unit electrolyte respectively; an anolyte circuit (9a, 9b) comprising a supply line (10a, 10b) and a return line (11a, 11b) connecting the anolyte tank (7a, 7b) to the anodes (3a, 3b) in the flow battery cells (2a, 2b) in the flow battery string; a catholyte circuit (12a, 12b) comprising a supply line (13a, 13b) and a return line (14a, 14b) connecting the catholyte tank (8a, 8b) to the cathodes (4a, 4b) in the flow battery cells (2a, 2b) in the flow battery string; and a state-of-charge measuring device (17a, 17b) capable of measuring the state of charge of the flow battery unit electrolyte in the flow battery unit (A1, B1); a flow battery string comprising at least one flow battery cell (2a, 2b) wherein each cell comprises an anode (3a, 3b), a cathode (4a, 4b), and flow battery unit electrolytes comprising an anolyte (5a, 5b) and a catholyte (6a, 6b); an anolyte tank (7a, 7b) and a catholyte tank (8a, 8b) for storage of the anolyte (5a, 5b) and the catholyte (6a, 6b) of the flow battery unit electrolyte respectively; an anolyte circuit (9a, 9b) comprising a supply line (10a, 10b) and a return line (11a, 11b) connecting the anolyte tank (7a, 7b) to the anodes (3a, 3b) in the flow battery cells (2a, 2b) in the flow battery string; a catholyte circuit (12a, 12b) comprising a supply line (13a, 13b) and a return line (14a, 14b) connecting the catholyte tank (8a, 8b) to the cathodes (4a, 4b) in the flow battery cells (2a, 2b) in the flow battery string; and a state-of-charge measuring device (17a, 17b) capable of measuring the state of charge of the flow battery unit electrolyte in the flow battery unit (A1, B1); a flow regulator in at least one of the anolyte and the catholyte circuits (9a, 9b, 12a, 12b) regulating flow of at least one of the anolyte (5a, 5b) and the catholyte (6a, 6b) therethrough in at least one of the flow battery units (A1, B1) in the at least two flow battery unit strings (SA, SB); a controllable bi-directional power converter electrically connected to each of the at least two flow battery unit strings (SA, SB); and a controller with inputs (20a, 20b) connected to each of the state-of-charge measuring devices (17a, 17b) in the flow battery units (A1, B1) and with an output (21a, 21b) connected to the flow regulator wherein the output of the controller controls the flow regulator so as to regulate the flow of at least one of the anolyte (5a, 5b) and the catholyte (6a, 6b) to the anodes (3a, 3b) and cathodes (4a, 4b) in the flow battery cells (2a, 2b) in the flow battery string in accordance with the states of charge of the flow battery unit electrolytes in the flow battery units (A1, B1) and thereby balance the state-of-charge in the at least two flow battery unit strings (SA, SB).