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

36 Focus | Battery technology power for 24/7 operation for months, but that means having batteries with an energy density of 1000 Wh/kg. That is being explored with silicon technology in the cell, but there are some reliability challenges to overcome. With such long mission times, there is also the added power drain of comms with the ground. The ultimate aim is to use multiple HAPS systems as a ‘cellular base station in the sky’ as a network to provide mobile phone and data connections to isolated regions of the world. However, this requires even more power for the wireless links between the craft, the links to the terminals or phones on the ground and a backhaul link to the terrestrial networks. That dramatically increases the energy density required of the battery. Other airborne systems have different power requirements. Users of a small quadcopter or octocopter for example want fast charging in a couple of minutes, which usually means the battery needs a larger anode and cathode to support the high incoming currents, but that leads to a heavier battery cell that reduces the flight time. Other applications, such as hybrid VTOL craft, have a mix of requirements. High power delivery is needed for a short time for the vertical take-off, while less power is needed after the transition to horizontal flight. Unmanned aircraft designers are reluctant to pay the weight penalty of just using high-power cells for 5% of the mission time for the vertical lift. These systems also need to be charged quickly to keep them operating, again leading to heavier cells. If they are used for unmanned air taxi applications for example then the safety of the passengers is paramount, again leading to heavier cells. Cell materials The vast majority of commercial battery cells for unmanned systems use a mixed metal oxide as the cathode with a carbon anode and lithium salt electrolyte, usually made of graphite. This has a higher energy density than previous technologies such as nickel metal hydride. Silver oxide and zinc have been used for battery cells for underwater applications such as unmanned targets, as the energy density is key for these remote acquisition target applications. While silver zinc provides packs with up to 7 kWh of energy, the cells can take up to 30 hours to charge and need to be refrigerated while doing so. These are now being replaced by lithium-ion battery packs that charge in 3 hours at room temperature. LFP is seen as more stable than other lithium technologies, but it has a lower power output. This technology is a lot less vulnerable to thermal runaway than other types of lithium cell. Thermal runaway is where a short-circuit causes heating in the cell, leading to more short- circuits and more heating until the cell bursts into flames. The short-circuits are caused by thin whiskers of lithium metal called dendrites, which form during the charging processes. The most popular cells use lithium with nickel, manganese and cobalt (NMC) or nickel, cobalt and aluminium (NCA) as the cathode, with a graphite anode. Using cobalt is an issue though, as there are limited sources of the metal and supply can be unreliable, so cell makers are researching ways to eliminate it from the cathode. NMC has an energy density of 205 Wh/kg, NCA is at 220 Wh/kg. They are used for high-power requirements in vehicles such as self-driving trucks and buses. NMC comes with various ratios of materials – NMC811, which is popular at the moment, is eight parts of nickel to one part manganese and one part cobalt, as this provides the highest energy density. April/May 2021 | Unmanned Systems Technology A comparison of different materials for battery cells (Courtesy of Amprius Technologies)