NOVELTY - The device has an electrochemical cell (102) comprising an anode electrode (102A) and a cathode electrode (102B). A vertically oriented reservoir (104) is configured to store an aqueous halogen electrolyte (114). A mass distribution measuring device comprises a scale, a first pressure sensor located in a lower portion (104A) of the reservoir and a second pressure sensor located in an upper portion (104C) of the reservoir. A strain gauge or load cell is configured to measure change in weight of the electrochemical cell. A pump (111) is located in a flow circuit.    USE - Electrochemical device i.e. flow battery (claimed) for off-peak energy storage in a power grid.    ADVANTAGE - The device allows the mass distribution measuring device to inherently compensate unpredictable efficiency losses such as dissipation in cables, waste heat produced in the cell, electrode plating corrosion and shunt currents in the electrochemical cell when the mass distribution measuring device directly measures amount of reactants that are available for electrochemical reaction. The electrochemical device is designed to eliminate factors such as leaks e.g. air bubbles generation, in production design even in the electrochemical system with sub optimal designs. The device can correct gradual fluid loss due to permeation or slow leak by periodically measuring state of charge with redundant measurement such as open circuit voltage, or even by using multiple types of mass measurement and increase efficiency of halogen reactant's reduction by using porous material in the anode electrode.    DETAILED DESCRIPTION - The aqueous halogen electrolyte e.g. zinc chloride and/or zinc bromide. The flow circuit i.e. closed loop circuit. The cell i.e. hybrid flow battery cell.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of an electrochemical system.    Electrochemical system (100)    Electrochemical cell (102)    Anode electrode (102A)    Cathode electrode (102B)    Vertically oriented reservoir (104)    Lower portion (104A)    Upper portion (104C)    Pump (111)    Aqueous halogen electrolyte (114)