# A Comprehensive Review of Micro UAV Charging Techniques
###### Abstract  
The groundbreaking Unmanned Aerial Vehicles (UAVs) technology has gained significant attention from both academia and industrial experts due to several applications, such as military missions, power lines inspection, precision agriculture, remote sensing, delivery services, traffic monitoring and many more. UAVs are expected to become a mainstream delivery element by 2040 to address the ever-increasing demand for delivery services. Similarly, UAV-assisted monitoring approaches will automate the inspection process, lowering mission costs, increasing access to remote locations and saving time and energy. Despite the fact that unmanned aerial vehicles (UAVs) are gaining popularity in both military and civilian applications, they have a number of limitations and critical problems that must be addressed in order for missions to be effective. One of the most difficult and time-consuming tasks is charging UAVs. UAVs' mission length and travel distance are constrained by their low battery endurance. There is a need to study multi-UAV charging systems to overcome battery capacity limitations, allowing UAVs to be used for a variety of services while saving time and human resources. Wired and Wireless Power Transfer (WPT) systems have emerged as viable options to successfully solve this difficulty. In the past, several research surveys have focused on crucial aspects of wireless UAV charging. In this review, we have also examined the most emerging charging techniques for UAVs such as laser power transfer (LPT), distributed laser charging (DLC), simultaneous wireless information and power transfer (SWIPT) and simultaneous light wave information and power transfer (SLIPT). The classification and types of UAVs, as well as various battery charging methods, are all discussed in this paper. We've also addressed a number of difficulties and solutions for safe operation. In the final section, we have briefly discussed future research directions. wireless power transfer; unmanned aerial vehicles (UAVs); charging; battery capacity; laser power transfer (LPT) +
Footnote †: journal: _micromachines_  
and sensing have captured the interest of several service providers including Walmart, DHL, Google and Amazon [2,3]. Due to the extensive increase in online shopping, users now require high-speed delivery services. However, fast delivery is becoming a successful tool for online retailers. Aside from that, retailers may face a critical challenge in providing eco-friendly, cost-effective and efficient last-mile delivery. In this regard, UAVs have emerged as promising solutions with fast and innovative concepts to ensure last-mile delivery along with environmental security. Moreover, drones have become a key element in 5G networks, either as a base station or a mobile relaying system to enable cellular UAV networks. With such an expanding and staggering market interest, service reliability of integrating UAVs will become a critical success factor. However, there are several crucial factors which limit the performance of UAVs. Some of these factors are limited battery endurance, restricted mobility, limited autonomy and limited flight time. Limited flight time is due to different elements, including sensor accuracy, harsh atmospheric conditions, fixed-wing size and battery endurance. Several studies have focused on multiple aspects of UAVs [4] and suggested viable solutions to overcome these challenges, such as using high-quality devices including batteries, wing, geometry, manufacturing materials and motors. Some studies have reported optimization algorithms to seek the shortest route for UAVs to reach their intended destination. Figure 2 shows that the UAV market is expected to reach $43 billion in total sales by 2025. UAVs are intended to be controlled remotely, either through predefined trajectories or through a radio controller. UAVs are now essential components in both civilian and military applications. UAVs, on the other hand, are energy-hungry devices that deplete batteries within minutes of operation.