A fluid-cooled battery system includes at least one battery module which includes a plurality of rows of battery cells, an outer casing, and at least one cell fixture. The outer casing defines therein an accommodation space. The cell fixture includes a holding web fitted inside the accommodation space, and formed with a plurality of rows of retaining holes. The retaining holes of each row are configured to retain cell bodies of a respective row of the battery cells so as to permit the battery cells to be held in the accommodation space by the holding web, to thereby keep the battery cells in stable position against undesired vibration.