This disclosure is directed toward traction battery packs that include energy absorbing structures for protecting the internal contents of the battery pack. An exemplary battery pack may include an enclosure assembly and one or more battery arrays that are housed within the enclosure assembly. The enclosure assembly includes an energy absorbing structure that may include a plurality of fluid filled unit cells. The stiffness of each fluid filled unit cell may be controlled by allowing fluid to be released from the unit cells during vehicle impact events, thereby absorbing impact energy that could otherwise be imparted inside the battery pack.