Structure-Battery Composites for UUVs: Multifunctional Interaction Effects

Abstract

Multifunctional structure-battery (SB) composites have recently been designed, fabricated and characterized for use in unmanned underwater vehicles (UUVs). This works reports on advanced multifunctional testing and performance of these composites. The SB composites described in this work use traditional fiber reinforced layers to provide structure function and commercial rechargeable lithium polymer “pouch” cells to provide both energy storage and structure function. SB composites can free-up interior hull space for additional fuel or other payloads through relocation of system batteries into skin and/or other structural components. Prior characterization of three fabricated SB beam prototype designs demonstrated similar bending performance, buoyancy, and dimensions to their unifunctional counterparts, and volumetric energy storage densities ranging from 42 to 58 Wh/L. Advanced multifunctional testing performed in this work shows the presence of load relaxation effects under constant-displacement 3-point flexure due to anelastic shear of the specimens' adhesive layers and the embedded battery cells. Up to 6% decrease in load was observed in the SB sandwich specimens. The relaxation is also affected by concurrent discharge-charge cycling of the embedded battery cells due to concomitant volumetric changes in the cells. Energy storage capacity under hydrostatic pressure was also measured and showed a decrease in capacity of ~6-8% with pressures up to 2 MPa. Additional testing is underway to assess the flexural strength of the SB prototypes and the combined effects of discharge rate and hydrostatic pressure on energy storage capacity.