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• A space-efficient protective method for battery pack is proposed. • Resin material offers high bearing capacity while foam improves energy absorption. • The protecting mechanisms of various matrix materials are revealed. • Stiffness balance between foam and battery for better protecting effect is unlocked. Mechanical protection of battery packs is challenging due to the limited available space. While filling packing gaps with protective structures is practical and space-efficient, there is a lack of reference for selecting proper matrix materials for the structures. To address this issue, we conducted compression tests on cylindrical battery packs embedded into 3D-printed thin-walled resin, foam, and polyurethane structures. A comprehensive comparison was made among battery packs protected by various materials, as well as the non-protected one, to evaluate their protecting performance including deformation nonuniformity, failure rate, and failure thresholds under various loading conditions. Results showed that the resin- and foam-protected battery packs experienced less damage under similar load level and similar crushing energy, respectively. The underlying mechanisms were further investigated by combining tests and simulations. It is revealed that the stiffness of filling material greatly influences the protective effect, which results in the different distributions of both stress and strain energy in the battery pack. The stiffness balance between the foam and the batteries to achieve better protecting effect was also unlocked under varying crushing energies. Our findings shed light on the space-efficient protection of battery packs under crash. [ABSTRACT FROM AUTHOR]