# A Comprehensive Review of Micro UAV Charging Techniques
## 3 Battery Charging Techniques for UAVs
### Photovoltaic Cell-Based UAV Charging  
PV cells are commonly considered to charge the batteries and enhance the flight time of UAVs. PV cells make use of sunlight to charge UAV batteries. Both the PV cells and the batteries utilized in a UAV are used in this way. Whenever sunlight is present, the required power is provided by PV cells and in the absence of sunlight, batteries are used to deliver the required power to the UAV. Research work on solar-powered UAVs reported that several parameters play key roles such as temperature intensity, angel of incidence of sunlight, geometry and the position of PV cells [70]. This technique is not suitable in scenarios with insufficient sunlight. There is a need to adopt alternative strategies whenever sunlight is not present to continue UAV flight. These methods include additional power supplies, enhanced battery power or PV cell size and automatic position adjustment according to sun position. As PV cells require a certain payload capacity and wing length of the UAV, it is feasible for fixed-wing UAVs. Moreover, environmental conditions such as humidity, temperature, fog and clouds reduce the system efficiency or reliability.  
Solar energy-based UAVs have gained the attention of many researchers. Figure 9 presents solar-powered UAVs. Solar-powered aircrafts are currently being used for power line inspection, forest fire fighting, border surveillance and high-altitude communication. This technology can be used to supersede environmental, scientific and communication satellites used for military and civilian applications. Sufficient flight power can be achieved through the appropriate selection of PV cells and taking into account the efficiency and weight is the key to ensuring long flight times and high endurance in solar-powered UAVs. Different solar cells and materials are used to attain cost-effectiveness and high efficiency. Some studies suggest mono-crystalline silicon PV cells due to their affordable cost and high efficiency [71]. Furthermore, mono-crystalline silicon PV cells offer high flexibility which supports easy integration in the UAV wing. It is still important to investigate PV cells with novel designs, energy management strategies and manufacturing materials to ensure high efficiency and high availability [72]. The research fraternity should focus more on material science developments as low photoelectric efficiency is a major limiting factor in this area [73]. Some researchers proposed an effective approach for the optimization of solar-powered UAV flight trajectories to gain more solar radiation, along with low consumption of mechanical energy [74].  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Type**} & **Energy** & **Power Density** & **Cycle Life** & **Efficiency of** & **Advantages** & **Drawbacks** \\  & **Density** & **(W/kg)** & **(Times)** & **Discharging (\%)** & **Advantages** & **Drawbacks** \\ \hline Lead-acid battery & 30–40 & 200–300 & 300–400 & 75 & High recycle rate, low cost & Poor performance at low temperature \\ \hline Ni-Mh battery & 60–80 & 800–1500 & \(>\)1000 & 75 & Long lifespan, high energy density & High manufacturing cost, high self-discharging rate \\ \hline Li-ion battery & 100–120 & 600–2000 & \(>\)1000 & 90 & Long cycle life, lightweight, high energy density, high voltage & Security risk, non-overcharge, life reduce at high temperature \\ \hline Super capacitor & 4–15 & 1000–10,000 & \(>\)10,000 & 85–98 & Fast charging and discharging speed, pollution-free and extremely long life & Low energy density \\ \hline \hline \end{tabular}
\end{table}
Table 3: Comparison between batteries and super capacitor [69].