An energy storage system (102, 300) comprising two energy storage modules stacked in a longitudinal direction, each of the energy storage modules comprising: at least one energy storage cell unit; at least one cooling plate unit stacked with said at least one energy storage cell unit in a longitudinal direction, each of said at least one cooling plate unit being in thermal contact with at least one of said at least one energy storage cell unit; and a first elongated retaining element arranged through a through-hole of each of the at least one energy storage cell unit and the at least one cooling plate unit in said longitudinal direction, said first elongated retaining element having a through-hole in an elongation direction of said first elongated retaining element, said through-hole of said first elongated retaining element being adapted to receive a second retaining element, said first elongated retaining element comprising a first end portion having a dimension larger than a dimension of said through-hole of said at least one energy storage cell unit or said at least one cooling plate unit such that said first elongated retaining element is prevented from passing through the through-hole of at least one of said at least one energy storage cell unit or at least one of said at least one cooling plate unit, wherein said first elongated retaining element comprises a deformed second end portion having a dimension larger than a dimension of said through-hole of said at least one energy storage cell unit or said at least one cooling plate unit such that said deformed second end portion is prevented from passing through the through-hole of at least one of said at least one energy storage cell unit or at least one of said at least one cooling plate unit, said first elongated retaining element being arranged to maintain a pressure on said energy storage module in said longitudinal direction such that said thermal contact is maintained, wherein said energy storage system (102, 300) further comprises: a second retaining element arranged through said through-hole of said first elongated retaining element of each of said two energy storage modules, said second retaining element being arranged to maintain a relative position of said energy storage modules in said longitudinal direction. at least one energy storage cell unit; at least one cooling plate unit stacked with said at least one energy storage cell unit in a longitudinal direction, each of said at least one cooling plate unit being in thermal contact with at least one of said at least one energy storage cell unit; and a first elongated retaining element arranged through a through-hole of each of the at least one energy storage cell unit and the at least one cooling plate unit in said longitudinal direction, said first elongated retaining element having a through-hole in an elongation direction of said first elongated retaining element, said through-hole of said first elongated retaining element being adapted to receive a second retaining element, said first elongated retaining element comprising a first end portion having a dimension larger than a dimension of said through-hole of said at least one energy storage cell unit or said at least one cooling plate unit such that said first elongated retaining element is prevented from passing through the through-hole of at least one of said at least one energy storage cell unit or at least one of said at least one cooling plate unit, wherein said first elongated retaining element comprises a deformed second end portion having a dimension larger than a dimension of said through-hole of said at least one energy storage cell unit or said at least one cooling plate unit such that said deformed second end portion is prevented from passing through the through-hole of at least one of said at least one energy storage cell unit or at least one of said at least one cooling plate unit, said first elongated retaining element being arranged to maintain a pressure on said energy storage module in said longitudinal direction such that said thermal contact is maintained, wherein said energy storage system (102, 300) further comprises: a second retaining element arranged through said through-hole of said first elongated retaining element of each of said two energy storage modules, said second retaining element being arranged to maintain a relative position of said energy storage modules in said longitudinal direction.