This paper investigates the mechanical properties of lithium-ion polymer (LiPo) batteries and their subsequent use in the design of multifunctional sandwich panels for automotive applications. Shear properties, flexural properties and compression properties of prismatic pouch LiPo batteries are determined experimentally through a hole-punch test, a three-point bending test and an in-plane compression test, respectively. This study is the first to characterize the shear properties of a lithium-ion battery, which are critical in sandwich panel design. The mechanical properties of the batteries obtained are then applied to existing analytical models of multifunctional sandwich panels consisting of carbon fibre composite facesheets and LiPo battery cores, which are currently being considered for use in automotive panel design. A material selection procedure for a stiffness-limited automotive car door panel subjected to bending shows that a trade-off between mechanical performance and cost can be achieved by using a composite sandwich panel with thin LiPo battery cores or by embedding larger LiPo batteries in lower-density polymer foam cores. The practicality and implementation aspects of using sandwich composites with LiPo battery cores in automotive design are also discussed.