A vanadium redox battery energy storage system (“VRB-ESS”) capable of modularly incorporating additional electrolyte reservoirs to increase energy capacity while allowing for efficient low-volume operation is disclosed. The VRB-ESS of the present invention may efficiently operate using a first volume of electrolyte solution, while maintaining a second volume of electrolyte solution to be made available to the VRB-ESS as additional energy storage capacity is required. Additionally, a cap mechanism to allow the VRB-ESS of the present invention to employ an industry standard IBC container as a secondary electrolyte reservoir is disclosed.