## 3 Embedded Battery Tile Design and Construction
### Embedded Battery Tile Prototype Design  
The proposed embedded battery tile prototype is shown in Figures 2 and 6 and descends from the preliminary design work on modular multifunctional structures for CubeSats [21]. The configuration is inspired to advanced sailplanes wing boxes concepts [22]. Four lithium polymer (LiPo) batteries are enclosed between two CFRP skins, as shown in yellow and red in Figure 6a. The skins, besides contributing to the flexural and torsional stiffness of the tile, provide containment and protection to the batteries from the external thermal environment. The printed circuit boards (PCBs, as shown in green in Figure 6a) provide the necessary circuitry for the electrical power system. The batteries terminals are soldered to the inner PCB, and to allow the passage of the terminals, the inner skin has two rectangular holes. The outer PCB is placed in the external side of the tile and hosts two solar cells, while the inner PCB is placed on the internal side of the tile and hosts electronic components. The tile dimensions are 97.8 \(\times\) 82.2 mm\({}^{2}\). The tile is joined to a commercial skeleton (Figure 1b) with two sets of four screws. The outer PCB and the outer skin have eight holes to allow the joining (Figures 2c and 6), whose diameter is 3 mm. The employed set of four holes depends on which face of the skeleton the tile is mounted on. The design requirements are slightly different from the ones for sailplane design. The main design requirement is the low cost requirement, typical for most CubeSat projects. Another relevant requirement, as mentioned in Section 2.1, is the maximum protrusion thickness of 6.5 mm (CubeSat mechanical requirement 3.2.3 [2] (p. 9)), imposed by the CubeSat form factor. The requirements led to the following design solutions:  
* Low level of structural integration. As mentioned in the introduction section, a low level of structural integration between the energy storage components and the structural components was selected. Functional separation is still present in the design  
Figure 6: Embedded battery views: (**a**) tile detailed exploded view, showing its components; (**b**) tile exploded view with a subdivision of the tile in the outer assembly, facing space and the inner assembly, facing the inner payload bay. Tile side length scale: 100 mm, tile thickness scale: 5 mm. The component thicknesses are as follows: batteries—4.5 mm, PCBs—0.8 mm, skins—0.23 mm, and solar cells—0.17 mm. at a component level, between the components that implement the energy storage function and the components that implement the structural function. The electrical power system components can be clearly identified and isolated from the structural components. Nevertheless, the whole battery tile assembly performs both the energy storage and the structural function. This is known in the literature as the first degree of structural integration [12] (pp. 3-4). Superior degrees of integration introduce an increasing component-level functional integration, up to the microscopic scale. Although these degrees of integration are not applicable with a low-cost design, they would allow a considerable mass and volume optimization. They could be considered for a very high performance, non-low-cost solutions;
* Lithium polymer (LiPo) batteries. COTS Lithium Polymer batteries were selected based on their energy capacity and dimensions, considering the energy capacity of state-of-the-art CubeSats (Figure 7). The maximum protrusion of 6.5 mm was relevant in the selection of the battery model. LiPol LP452540 batteries were selected [23], with a nominal charge capacity of 480 mAh at 3.7 V, and thus a single battery capacity of 1.78 Wh. Four batteries were mounted on the embedded battery tile, with a total tile capacity of 7.12 Wh. This electrical energy capacity is slightly inferior to the typical capacity of state-of-the-art 1U CubeSats (Figure 7). However, since the smart tiles are modular, two or more embedded battery tiles can be mounted on the skeleton.