Traction battery pack assembling method

A battery pack assembly method includes compressing a first cell stack, compressing a second cell stack, and inserting the first cell stack together with the second cell stack into a cell-receiving opening of an enclosure structure. Another traction battery pack assembly method includes simultaneously inserting a plurality of separate cell stacks into an enclosure structure when the plurality of cell stacks are each compressed by a compression fixture and, after the inserting, compressing the plurality of cell stacks using the enclosure structure.