NOVELTY - Method for operating an electrochemical cell (10) comprising first electrode (14), second electrode (12), charging electrode (15), and ionically conductive medium (18) comprising reducible metal fuel ions and additive comprising poly(ethylene glycol)tetrahydrofurfuryl, the method comprises: charging the electrochemical cell by applying electrical current between the charging electrode and first electrode, such that reducible metal fuel ions are reduced and electrodeposited as metal fuel in oxidizable form on first electrode, and removing the electrical current to discontinue the charging.    USE - The method is useful for operating an electrochemical cell, including charging the electrochemical cell and generating electrical current (all claimed).    ADVANTAGE - In the method the electrochemical cell utilizes poly(ethylene glycol)tetrahydrofurfuryl which: enhances reduction of reducible metal fuel ions for electrodeposition as a metal fuel on a fuel electrode having a cathodic potential during the electrodeposition; or delays passivation of the metal fuel during oxidation with the fuel electrode having an anodic potential.    DETAILED DESCRIPTION - Method for operating an electrochemical cell (10) comprising a first electrode (14), a second electrode (12) spaced apart from the first electrode, a charging electrode (15), and an ionically conductive medium (18) communicating the electrodes, the ionically conductive medium comprising reducible metal fuel ions and an additive comprising poly(ethylene glycol)tetrahydrofurfuryl, the method comprises: charging the electrochemical cell by applying an electrical current between the charging electrode and the first electrode with the charging electrode functioning as an anode and the first electrode functioning as a cathode, such that the reducible metal fuel ions are reduced and electrodeposited as metal fuel in oxidizable form on the first electrode, and removing the electrical current to discontinue the charging. INDEPENDENT CLAIMS are also included for:    (1) an electrochemical cell comprising: a first electrode for oxidizing a metal fuel during discharging and reducing reducible ions of the metal fuel during recharging; a second electrode spaced apart from the first electrode for at least reducing an oxidizer during discharging; a charging electrode for oxidizing oxidizable ions of the oxidizer during recharging; an ionically conductive medium communicating the electrodes, the ionically conductive medium comprising reducible metal fuel ions and an additive comprising poly(ethylene glycol)tetrahydrofurfuryl; and    (2) an ionically conductive medium for use in a rechargeable current producing electrochemical cell comprising: a liquid comprising ions for supporting oxidation and reduction reactions at the electrodes of the cell; and an additive comprising poly (ethylene glycol)tetrahydrofurfuryl for improving cell performance.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of an electrochemical cell with a stack of permeable electrode bodies for generating electricity.    Electrochemical cell (10)    Second electrode (12)    First electrode (14)    Charging electrode (15)    Ionically conductive medium (18)