A metal halogen battery construction comprises at least one cell having a positive electrode and a negative electrode for contact with aqueous electrolyte containing the material of said metal and halogen, a sump (22) wherein the electrolyte is collected, a store (30) wherein halogen hydrate is formed and stored as part of an aqueous material, circulation means (P1) for transmitting electrolyte through the battery, a conduit (54) for transmitting halogen gas formed in the cell to hydrate forming means associated with the store (30), a fluid jet pump (50) operative to cause circulation of gases from the sump including hydrogen and halogen to contact a reactor means whereby hydrogen and halogen are combined and returned to the sump, and a valve (60) sensitive to gas pressure in the sump, whereby when sump pressure drops below a desired setting of said valve, the valve opens and permits halogen gas from the store to be transmitted to the electrolyte circulation means.