A redox flow battery having: • at least one redox fuel cell, the cell having: • an anode in a catholyte compartment, • a cathode in an anolyte compartment and • an ion selective membrane separator between the compartments, • a pair of electrolyte reservoirs, i.e. one for anolyte and the other for catholyte, • electrolyte supply means (9,10,11,12; 37,38,39,40,41,42), i.e. means for circulating anolyte from its reservoir, to an anolyte compartment in the cell and back to its reservoir and like circulating means for catholyte and • electrolyte withdrawal and supply connections (21,22,23,24; 35,36) to its electrolyte reservoirs and/or its electrolyte supply means whereby the battery can be recharged by withdrawing spent electrolyte and replacing it with fresh electrolyte; and wherein each electrolyte reservoir is provided with: • a movable divider for dividing the reservoir into two volumes, and one of the electrolyte withdrawal and supply connections is connected to one end of the reservoir and another electrolyte withdrawal and supply connection is connected to the other of the reservoir, whereby for recharging, the divider traverses along the reservoir with spent electrolyte being withdrawn from in front of the divider via one connector and fresh electrolyte being introduced behind it via another connector, with no communication between the spent and fresh electrolyte OR • a rigid outer vessel and two separate, collapsible tank liners (201,202), the liners having at least two unions passing through the wall of the rigid outer vessel, and one of the electrolyte withdrawal and supply connections is connected to one end of the reservoir and another electrolyte withdrawal and supply connection is connected to the other of the reservoir, whereby for recharging one of the liners collapses as spent electrolyte is withdrawn from it via one connector and the other is filled via another connector with fresh electrolyte to substantially the internal capacity of the outer vessel. • at least one redox fuel cell, the cell having: • an anode in a catholyte compartment, • a cathode in an anolyte compartment and • an ion selective membrane separator between the compartments, • an anode in a catholyte compartment, • a cathode in an anolyte compartment and • an ion selective membrane separator between the compartments, • a pair of electrolyte reservoirs, i.e. one for anolyte and the other for catholyte, • electrolyte supply means (9,10,11,12; 37,38,39,40,41,42), i.e. means for circulating anolyte from its reservoir, to an anolyte compartment in the cell and back to its reservoir and like circulating means for catholyte and • electrolyte withdrawal and supply connections (21,22,23,24; 35,36) to its electrolyte reservoirs and/or its electrolyte supply means whereby the battery can be recharged by withdrawing spent electrolyte and replacing it with fresh electrolyte; and • a movable divider for dividing the reservoir into two volumes, and one of the electrolyte withdrawal and supply connections is connected to one end of the reservoir and another electrolyte withdrawal and supply connection is connected to the other of the reservoir, whereby for recharging, the divider traverses along the reservoir with spent electrolyte being withdrawn from in front of the divider via one connector and fresh electrolyte being introduced behind it via another connector, with no communication between the spent and fresh electrolyte OR • a rigid outer vessel and two separate, collapsible tank liners (201,202), the liners having at least two unions passing through the wall of the rigid outer vessel, and one of the electrolyte withdrawal and supply connections is connected to one end of the reservoir and another electrolyte withdrawal and supply connection is connected to the other of the reservoir, whereby for recharging one of the liners collapses as spent electrolyte is withdrawn from it via one connector and the other is filled via another connector with fresh electrolyte to substantially the internal capacity of the outer vessel.