A battery assembly comprising: a plurality of battery cells arranged in a first direction; a separator disposed between the plurality of battery cells; and a restraint member that restrains the plurality of battery cells and the separator along the first direction, wherein: - the separator includes a first member including a base portion and a plurality of protrusions each protruding from the base portion in the first direction, and a second member disposed between the plurality of protrusions, the second member having a higher heat insulation property than a heat insulation property of the first member, - -in a state in which the separator is detached from the battery assembly, a protruding height of each of the plurality of protrusions is larger than a thickness of the second member, characterised in that the first member and the second member are selected such that: - the first member is composed of a material having an elastic modulus of about 1 MPa or more and 10 MPa or less, - the second member has a lower elastic modulus than the first member, and - when a load is applied to the separator and its thickness is reduced to between 50% and 90% of its thickness in an unloaded state, the separator undergoes elastic deformation, and the load acting on the first member is greater than the load acting on the second member. a plurality of battery cells arranged in a first direction; a separator disposed between the plurality of battery cells; and a restraint member that restrains the plurality of battery cells and the separator along the first direction, wherein: - the separator includes a first member including a base portion and a plurality of protrusions each protruding from the base portion in the first direction, and a second member disposed between the plurality of protrusions, the second member having a higher heat insulation property than a heat insulation property of the first member, - -in a state in which the separator is detached from the battery assembly, a protruding height of each of the plurality of protrusions is larger than a thickness of the second member, - the separator includes a first member including a base portion and a plurality of protrusions each protruding from the base portion in the first direction, and a second member disposed between the plurality of protrusions, the second member having a higher heat insulation property than a heat insulation property of the first member, a first member including a base portion and a plurality of protrusions each protruding from the base portion in the first direction, and a second member disposed between the plurality of protrusions, the second member having a higher heat insulation property than a heat insulation property of the first member, - -in a state in which the separator is detached from the battery assembly, a protruding height of each of the plurality of protrusions is larger than a thickness of the second member, - the first member is composed of a material having an elastic modulus of about 1 MPa or more and 10 MPa or less, - the second member has a lower elastic modulus than the first member, and - when a load is applied to the separator and its thickness is reduced to between 50% and 90% of its thickness in an unloaded state, the separator undergoes elastic deformation, and the load acting on the first member is greater than the load acting on the second member.