A method for upgrading used or rejected electric battery cells, which include upgradable compounds, such as iron, zinc, manganese, copper, and fixed and volatile carbon, and heavy metals and dangerous compounds. The used or rejected battery cells are introduced as a load into a furnace for melting metal, such as a cupola furnace, a free arc furnace, or an induction furnace. A device for purifying gases produced by the furnace and for capturing and removing noxious elements, such as mercury, chlorides, and fluorides, and heavy molecules such as dioxins, furans, and aromatic substances, is provided in a discharge route of the hot gases, downstream from the melting furnace.