Unmanned Systems Technology 004 | Delair-Tech DT18 | Autopilots | Rotron RT600 | Unmanned surface vehicles | AMRC | Motion control | Batteries

75 The structure also includes vents to release gases generated by ion exchange. The other type, lithium-polymer (LiPo) cells, have become more popular in recent years. These use a polymer electrolyte in a pouch that can be moulded into different shapes. This is a lighter design as it leaves out the protective layers of the cylindrical cell, and its use in different shapes can help unmanned system designers to use more of the available space to provide power. However, if LiPo cells are discharged too fast, this can lead to overheating and to the cells catching fire when in use. For example, in 2014, fire officers in the Netherlands were monitoring hay fires using small UAVs that were themselves catching fire from the batteries and causing fires of their own. There have also been problems with transporting LiPo cells so, for example, United and Delta airlines have banned bulk lithium battery shipments on their passenger planes. Ultimately, adding protective layers and more monitoring and smart charging technology to the cells to prevent over-discharge can reduce the weight benefit that LiPo pouches provide. New materials As a result of these challenges, there is a wide range of projects exploring the options for new types of Li-ion battery with different combinations of materials. All battery cells have a positive anode and a negative cathode. In present Li-ion systems the cathode is made from cobalt oxide and the anode is graphite (carbon), as this combination gives a high energy density. However, it also delivers a low discharge current to prevent the battery overheating and catching fire. New materials being developed for the cathode include iron oxide and manganese oxide, and even a conducting ‘glass’ made of vanadium and boron to give a higher discharge rate and an energy density of up to 1000 Wh/ kg for the vanadium-boron silicate. By contrast, the Li-ion cells used in the Tesla Model S have an energy density of 245 Wh/kg, which provide 160 Wh/kg to Batteries | Insight people The lithium-ion cells used in the Tesla Model S provide 160 Wh/kg to the motors after losses (Courtesy of Tesla Motors) Unmanned Systems Technology | Autumn 2015