Abstract: In an electric cell stack assembly, each cell (1) in the top cell stage of the stack abuts against a holder member (7) in the very top stage of the stack having substantially the same mechanical characteristics, relative to a cell of the top cell stage, as has each cell relative to a cell of the next lower stage in the stack. Reinforcing members (8, 9) run along the entire length of the stack, including the said very top stage. The assembly is inserted in a tube and is encapsulated in a compound which preferably has a useful temperature range extending from -55.degree. C. or less to +250.degree. C. or more.