A non-hermetically sealed, electrochemical power source, includes a first electrode, a second electrode, a separator between the first electrode and the second electrode, and a membrane in fluid communication with an environment external to the battery. The second electrode is between the separator and the membrane. The membrane includes a first portion having a different property, e.g., density, porosity, mass transport resistance, thickness, or gas permeability, than a second portion of the membrane. Methods of designing an electrochemical cell cartridge are also disclosed.