## 2 Materials and Methods
### Modifications Performed to Original ALO RPA
#### 2.2.3 New Electronic Power System  
Battery and BMS  
Taking into account a typical mission power needs, and the expressions for energetic content and capacity of a battery [75], the final battery pack was composed by 28 commercial 16,000 mAh 15 C 3.7 V Lithium Polymer battery cells [76] with 14-series and 2-parallel configuration (14s2p), achieving a final pack with 32,000 mAh theoretical capacity and 58.8 V [77; 78]. For the selected electric motor, the different types of commercial batteries available were taken into account: Acid lead, NiMH, NiCd, LiPo, and LiFePO4, although new materials for batteries in aerospace and aeronautical applications will be available in the coming years [79; 80; 81]. However, with the actual state of technology and market prices, it was concluded that the best battery to be used in the electric ALO is the lithium polymer (LiPo) battery. It is the one with the highest specific energy, despite being the most fragile. Its main advantages are reflected below:  
1. High voltage per cell. In this case, the nominal voltage of each cell is 3.7 V, and can reach a maximum value of 4.2 V.
2. High specific energy, around 200 Wh/kg. 3. Low weight. Figure 6: Controller 2 software screen with description and the possible values for all the parameters. * High power density. * Fast charge. * Long life, which translates into economic savings. * Passivation phenomenon: it consists in the formation of a lithium chloride film that forms on the surface of the anode and prevents its self-discharge. This is beneficial if the battery is not used for a long period of time, as it prevents it from discharging. In contrast, some disadvantages are:  
* Degradation at temperatures higher than 60 \({}^{\circ}\)C.
* Degradation if discharged below its minimum voltage, 3 V.
* Because of the two previous limitations, it is essential to use a BMS (battery management system), which controls the operating range of the batteries, and is able to disconnect them if their operation exceeds the permitted thresholds. The amount of energy, E, stored in the battery is expressed in watt.hours (Wh) and is represented by Equation (1):  
\[E=\int_{0}^{t}U\left(t\right)*I\left(t\right)dt \tag{1}\]  
Usually when calculating this value, a constant current discharge mode can be used in such a way that the only variable in the equation is the voltage. The specific energy is measured in watt-hours per kg (Wh/kg) and it should be as high as possible since in mobile applications the weight is a key factor. This parameter will then be decisive when deciding what type of battery will be used in the ALO, which will be introduced later. The battery capacity, \(C_{Ah}\), is defined internationally as the electric charge, in amp hour (Ah) units, which can be extracted from a battery. When a battery is discharged at a non-constant current, the capacity is given by Equation (2):  
\[C_{Ah}=\int_{0}^{t}I\left(t\right)dt \tag{2}\]  
The capacity depends on discharge parameters, as well as: (1) The discharge current, (2) limits imposed by the voltage (cut-off voltage, final discharge voltage, initial voltage, state of charge, etc. ), and (3) operating temperature. When reaching the necessary voltage or achieving the desired capacity, the cells must be connected in series, parallel, or a combination of both. The serial connection consists of connecting the positive and negative terminals of two cells in such a way that the voltage increases, although the capacity remains the same. The parallel connection consists of connecting the terminals of the same sign (positive with positive and negative with negative) in such a way that the total voltage remains the same, but the capacity doubles. When selecting the configuration to be used in the application, it is necessary to take into account the voltage first and finally the capacity. In this case, the named 14-series and 2-parallel (14s2p). The cells were packed in a custom 3D printed case (Figure 7) with all the couplings and fasteners secured in the main bay of the RPA.