Issue 37 Unmanned Systems Technology April/May 2021 Einride next-gen Pod l Battery technology l Dive Technologies AUV-Kit l UGVs insight l Vanguard EFI/ETC vee twins l Icarus Swarms l Transponders l Sonobot 5 l IDEX 2021 report

35 Use cases Much of what is required from a battery depends on its usage, and the power requirements of unmanned systems are growing all the time. For one thing, the systems are being designed with more sensors that need more power to provide longer range detection, particularly with radar or Lidar. Rotating radar or Lidar turrets for 360 º sensing for example can take significant amounts of power. But an unmanned system can have other requirements than just its own power. Defence applications for example need a longer ‘silent watch’ capability that requires more power for other equipment from batteries rather than noisy diesel generators. However, that can amount to as much power as is required for moving the entire vehicle, with tens of kilowatts needed for 8 to 12 hours. The silent watch provision requires a lower power delivery than for the motors, which may need to provide high peak power for fast manoeuvrability, especially when there is no need to worry about any people on board. Defence applications also need to consider the ruggedness of the batteries, despite the lack of people in the vehicle. Battery packs for military vehicles undergo penetration testing to ensure they are safe, and indeed will still operate if compromised by a projectile. Some lithium-ion cells with liquid electrolytes for example can short under such conditions and catch fire, compromising the rest of the battery pack. Conformal cells These design issues lead to trade-offs in the protection of the battery pack, with heavier protection and the intrinsic safety and performance of the battery materials in the cells. In turn, this leads to a consideration of the overall energy density of the unmanned vehicle. Without an operator on board, there is potentially more space for battery cells in different places in the vehicle, leading to changes in the overall design. Prismatic or pouch cells can be designed in specific shapes to fit parts of the structure of the vehicle, such as the side panels. This provides higher total power for the vehicle but its architecture is more complex to charge and to replace the batteries than using a traditional modular battery pack, as it would require additional wiring and possibly controllers for the cells distributed around the vehicle. One example of a conformal cell is a flexible lithium-ion cell about 0.5 mm thick that can be made in various sizes and embedded into objects such as composite panels, although it has a lower specific energy than other cell formats. Battery cells can also be used as part of a vehicle’s structure rather than being a separate pack. This changes the choice of materials used for the battery cell, for example by using a thicker steel for a cylindrical cell that is then part of the vehicle. This can allow the removal of steel from other parts of the vehicle chassis and so increases the overall energy density of the design. Some materials for battery cells, such as carbon fibres, are even used in the panels of the vehicle itself. These can be integrated into a fibreglass panel as the anode with the electrolyte, with similar performance to discrete cells. The key is to make the battery a structural item rather than turning a structural element into a battery. With the development of solid- state battery cells, vehicle developers are experimenting with building the batteries as the panels themselves. These conformal cells, which are being developed with Kevlar coatings for a thin layer of protection, can be used to boost the energy available to the vehicle, but come at a much higher cost as each cell is a custom design. Airborne designs There are very different power requirements for airborne vehicles. For example, lightweight systems such as high altitude pseudo satellites (HAPS) have a total focus on the energy density. These craft are powered by solar cells that charge the batteries during the day to release power overnight to stay in position, at altitudes of 70,000 ft. The first versions of these systems used lithium sulphur cells with an energy density of 400 Wh/kg to power the motors and cameras, and give the system a mission time of 10-12 days. The aim is to provide enough Battery technology | Focus Unmanned Systems Technology | April/May 2021 Silver oxide zinc batteries are used in unmanned underwater systems but are now being replaced by lithium-ion (Courtesy of Saft)