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

38 Using NMC or NCA cells allows developers to move between manufacturers and chemistries. NMC is used for its energy density, which is higher than with LFP, and has a longer cycle life of 2000 cycles to 80% state of charge. Using open-source wire bonding techniques to connect the individual cells, whether they are NMC or NCA, provides cell level-fuses. The aluminium wire bond pops like a fuse and disconnects the cell in the event of a short-circuit, protecting the rest of the pack. The length, material and diameter of the bond is engineered for the cell and application. With this technology, the quality of the wire is vital, and it needs regular inspection and testing. However, the design of an unmanned system is also affected by the materials used in the battery pack. A large unmanned ground vehicle for example needs a 20 kWh battery pack, which is too heavy to replace in one go, so a modular design with four, 5 kWh packs is used. That allows a pack weighing 50 kg to be easily removed by two people, and for the system to keep running if one or more packs fail. It also means the battery management system (BMS) has to be able to balance the current and voltage from each pack during charging and discharge. It can then adjust the parameters for NMC or NCA cells. Silicon Silicon is a key material for battery cells, as it boosts the capacity of the carbon anode. This approach provides an energy density of 500 Wh/kg.  However, silicon swells during the charging process, increasing in size by a factor of four. That can cause the cell to split and leak during charging, potentially also catching fire. Different techniques are used to prevent the swelling, ranging from creating smaller particles of silicon (nanoparticles) with a diameter of 100 nm, combining the silicon particles with different types of carbon such as nanotubes, or creating a scaffold of nanomaterials to hold the silicon particles safely to prevent the swelling. Another approach is to use silicon nanowires. These tolerate cell expansion and are attached to the substrate in such a way to accommodate the swelling. This achieves an anode thickness half that of a graphite electrode, with a resulting increase in energy density. Research laboratories around the world are looking into ways to combine silicon with carbon in the anode. Another key area of development is producing the silicon nanomaterials in volume with sufficient quality and the ability to fit into existing battery manufacturing techniques to keep costs down. Germanium, which is in the same family as silicon, is also being used to build a lightweight pouch cell on a commercial production line and with a fast-charging capability. The germanium nanoparticles and self-healing polymers allow a cell that can fully charge in as little as a minute, although that reduces its lifetime to 30 cycles, while a full charge in 5 minutes gives a lifetime of 150 cycles. Sixty-four of the cells have been combined in a pack with wireless charging for autonomous UAV operation, giving 30 minutes of flight time. The major difference with silicon is that germanium can be mixed with water, while silicon nanoparticles cannot as they react with it. This has meant developing organic solvents for the slurry containing silicon nanoparticles used in building the cells. Solid state Solid-state materials are expected to be the next big advance in battery technology. They use ceramic materials that allow the ions to move, and so support fast charging in minutes but without the risk of fire. This allows lithium metal rather than graphite or silicon to be April/May 2021 | Unmanned Systems Technology The Argo unmanned vehicle uses four modular lithium-ion battery packs for easier replacement (Courtesy of Vanguard Power)