# Power Sources for Unmanned Aerial Vehicles: A State-of-the Art
## 2 Unmanned Aerial Vehicle (UAV)
### Battery-Based Supply Techniques
#### 2.2.2 Swapping Method Algorithm  
The technique known as swapping is employed to recharge the batteries of a UAV while it is in the midst of its mission. This process involves the exchange of depleted batteries with fully charged ones, which can be performed either autonomously or with human intervention. A specific type of swapping, called hot swapping, enables the immediate replacement of a depleted battery with a fully charged one, allowing the UAV to maintain power and continue its operation without interruption. Subsequently, the UAV can return to its assigned operational area and resume its tasks. By deploying multiple UAVs and coordinating their actions, a multi-agent system can ensure uninterrupted coverage of a specific area [34]. To successfully carry out a swapping operation, three important conditions must be fulfilled: (1) the presence of a ground recharge station where UAVs can land for battery charging or replacement, (2) a group of UAVs capable of continuous operation, and (3) a management system that facilitates cooperation among the swarm of UAVs. Figure 4 illustrates the techniques of swapping and hot swapping. In the infrastructure network for UAVs, ground stations (GS), are strategically placed in various locations, such as cities or along connecting paths between cities. These GS can be installed on different structures like cell towers, streetlights, rooftops, power poles, or standalone pylons. As part of this infrastructure, battery swap stations are established, which consist of several components including ground electronics, an onboard circuit, a landing frame, and a contact mechanism. The batteries can be recharged through contact-based pathways or by utilizing inductive coupling. The docking platforms at the swap stations can be powered through a combination of power lines, large batteries, and solar cells, especially for stations located in remote areas. There was also research that developed a multi-rotor aerial prototype specifically designed for long-duration missions using the swapping approach. The main goal of their study was to ensure the uninterrupted operation of the airborne platform by monitoring the battery's state of charge (SOC). Their concept involved having three quadcopters in a rotating system, with one quadcopter always in a loiter position. When the SOC of a quadcopter drops below a predetermined threshold, it is replaced by another quadcopter, allowing the former to return to the ground station for battery charging. This cycle continues until all batteries are depleted or the mission is completed. The number of batteries required  
Figure 4: The swapping and hot swapping algorithms. for sustainable operation depends on the discharge time and the duration needed for charging, rather than the number of UAVs. However, a larger number of batteries increases the system's resilience and reliability [34, 35, 36]. However, it is important to mention that the process of battery swapping and charging on the launch platform was not automated and still relied on human intervention. Additionally, the field test conducted in the study did not involve specific missions that required the use of sensors; instead, only static loitering was performed. In a separate study, researchers conducted an economic comparison of battery refilling and recharging platforms. They developed three stations based on axiomatic design principles, which allowed them to analyze and establish relationships between cost, complexity, and coverage levels [78]. This analysis provided valuable insights into the various factors associated with the implementation of battery refilling and recharging platforms. The comparison between refilling and exchange stations has indicated that refilling stations are suitable for low-coverage scenarios, while exchange stations are preferable when higher coverage levels are required. In [79], research was conducted on a comparison of battery charging and replacement systems using a Petri net model.