Abstract: A fluid accumulator (1), preferably for use in connection with a hydraulic system which is to receive relatively large quantities of working fluid in a short period of time comprises a housing (2) and a piston (3) arranged therein, which separates the working fluid, especially an oil, from an expanding fluid, especially a gas, and which easily can be displaced in the housing (2) in dependence of the pressure conditions arising from the hydraulic system by being adapted to float freely between the two fluid phases by providing between the outer surface of the piston (3) and the inner wall of the housing (2) a space (7c) in which the two fluid phases can communicate freely and which at the same time provides a fluid sliding layer so that the piston (3) can quickly and effectively move between its various positions. The piston (3) has the shape of a cylindrical cup having a relatively large extension in the axial direction and having its opening facing upwardly.