Abstract: A metal oxygen-hydrogen battery having sealed cell modules with electrolyte containment and hydrogen venting. The battery includes an outer pressure vessel containing a plurality of cell modules, each having a positive and negative terminal and containing a liquid electrolyte such as potassium hydroxide. Each cell module is enclosed in a flexible sealed bag which is impervious to the flow of the electrolyte. The bag is a multi layer structure preferably composed of thermoplastic film. Each bag is provided with a vent which is permeable to the flow of hydrogen gas but impermeable to the flow of the electrolyte. Electrical leads are connected to the terminals of each cell module and extend in sealed relation through the bag.