A method of controlling water balance in a metal-air electrochemical cell during operation of the electrochemical cell involves controlling a ratio of partial pressure of water vapor in an inlet supply of air to equilibrium water vapor pressure of an electrolyte of the metal-air electrochemical cell. Controlling the ratio is done by controlling one or both of the partial pressure of water vapor in the inlet supply of air and the equilibrium water vapor pressure of the electrolyte. The method can be performed by a system having a programmable controller programmed to perform the method and configured to receive signals from sensors and control the relative humidity and temperature control devices based on the signals received from the sensors.