# A Comprehensive Review of Micro UAV Charging Techniques
## 2 Related Works  
Despite all the appealing applications of UAVs, battery-powered UAVs have a major drawback in limited flight duration. This challenge occurs due to the limited capacity of lithium-ion polymer (LiPo) batteries. Most of the previous reported works on UAV issues, such as resource allocation and 3D trajectory optimization, focuses on energy consumption when communicating, data processing, hovering or flying. Several research studies have focused on designing energy-efficient operational mechanisms such as UAV location or path optimization, battery capacity increase and battery hot-swapping to address the energy limitation issues. These mechanisms include communication and trajectory planning for UAVs [6; 8]. UAVs consume their battery power during communication, hovering and flying. However, UAVs consume more energy during hovering and flying compared to communication tasks. Thus, the research fraternity has focused on investigating optimization strategies to reduce flight trajectories [9; 10]. One approach that is being used to recharge is tethered UAVs [11]. In this technique, UAVs are linked to a base station through a cable to receive power supply continuously. However, this method is only useful \begin{table}
\begin{tabular}{c c c c} \hline
**EMF Based Charging** & **Charging Type** & **Non-EMF Based Charging** & **Charging Type** \\ \hline Capacitive charging & Static charging up to a few mm & Gust soaring & In-flight charging \\ \hline Inductive charging & Static charging up to a few cm & PV integrated & In-flight charging \\ \hline Magnetic resonance charging & Static charging up to a few cm & Laser beaming & In-flight charging \\ \hline \end{tabular}
\end{table} 
Table 1: Charging techniques to charge UAVs [4]. Figure 2: Statistics of Drones and WPT market growth [7]. in those locations where base stations are located and it also restricts the mobility of the UAVs. Another widely used approach to recharge UAVs is battery swapping [12]. In this technique, UAVs are required to fly back to the ground base stations to replace drained batteries with fully charged batteries. This approach is better than tethered UAVs because it is faster. However, this approach raises some serious concerns, such as the need for human assistance to replace batteries and the disruption of the UAV mission due to the need to leave the operational location to swap batteries. Another technique is called battery dumping, in which UAVs are equipped with many batteries and an individual battery is dumped when it is drained. This technique enhances the flight time by reducing the payload of UAVs. Another method of recharging UAVs is known as gust soaring where UAVs gain energy from airflow and wind through the principles of dynamic soaring. To overcome mobility and deployment location challenges, WPT techniques have emerged as promising alternatives. Researchers and industrial experts have focused on different trials, activities and projects related to WPT techniques. For instance, a New Zealand-based startup company, Emrod, has designed a high power, long-range WPT system to deliver wireless electricity to consumers without requiring copper power lines [13]. Following this idea, the Powerco company started trials of this technology in 2021 [14]. Furthermore, Laser-Motive, a US based company, demonstrated laser-based WPT to fly a drone for more than 12 h [15]. According to [16], it is envisaged that WPT will generate 12 billion dollars of revenue in 2020. This prediction can be seen in Figure 3. In the view of a 2020 Bloomberg report, the expected market value of the global drone wireless charging and infrastructure market for drones will hit USD 249.3 million by 2024 [17]. This is due to diverse applications in the electronic industry with several key benefits in terms of safety, convenience, reliability and a fully automated charging mechanism.