# Preliminary Propulsion and Power System Design of a Tandem-Wing Long-Range eVTOL Aircraft
## 4 Power System Design Methodology
### Battery Configuration  
Not only is the size of the battery important for the design, but so are the specifications in terms of configuration. A battery consists of a number of battery cells. The number of cells depends on the properties of the individual cells and the required properties of the total battery. Some parameters other than total battery size play a role in the preliminary design of the batteries, of which safety and ease of work are used in the preliminary design phase. A method that is considered to increase the safety consists of adding two motor controllers per motor, instead of one, allowing a redundant construction. However, duplicated motor controllers would result in a significant weight penalty given that each controller weighs approximately 11 kg based on the power density of [17], thus resulting in a total weight increase of approximately 130 kg. In addition to this, the Wigeon eVTOL was designed to withstand a single motor failure, and thus the weight penalty was viewed as too high to accept additional redundancies. The original analysis for the OEI condition can be found in [3]. Therefore, the main strategy adopted to achieve the safety of the power system for the present design consists of limiting the impact of battery failure. By having two batteries per electric motor, such redundancy can be achieved, such that in the case of a battery failure, the affected motor still has half of the original energy at its disposal. By assuming that all the battery packs are equal, a modular design can be achieved, making the aircraft easier to manufacture and maintain. The aircraft has an additional, dedicated battery for the non-propulsive parts of the power system, but this battery is not the focus of the preliminary design methodology presented herein. There are two ways of connecting batteries: in series or in parallel. Batteries connected in series have the same current, and the total voltage of batteries in series is the sum of their individual voltages, represented by Equation (18). Batteries connected in parallel, on the other hand, experience the same voltage and the total current is the sum of their individual currents. Equation (19) gives the total energy \(E\) stored in a single battery cell. This is found by multiplying the capacity \(C\) and the nominal cell voltage \(V\). The values for \(C\) and \(V\) have a major influence on the relative increase in the number of cells. This effect is described in more detail in Section 5.4. The values for \(C\) and \(V\) were chosen to minimise this increase:  
\[V_{tot} =\sum_{i=1}^{n}V_{i} \tag{18}\] \[E =CV \tag{19}\]  
Equation (20) was used to calculate the number of cells required for the electric motors only based on the required energy for the propulsion system. Equation (21) is used to calculate the number of cells for the remaining batteries. These values have to be rounded up to the next integer:  
\[n_{c_{mat}}=\left\lceil\frac{E_{tot}\times\%_{mat}}{100E_{c}}\right\rceil \tag{20}\]  
\[n_{c_{mic}}=\left\lceil\frac{E_{tot}\left(100-\%_{mat}\right)}{100E_{c}}\right\rceil \tag{21}\]  
where \(n_{c}\) is the number of cells, \(\%_{mat}\) is the percentage of energy that goes into the motors and \(E_{c}\) is the energy stored in a single cell. These two are separated as a consequence of the battery configuration, since both the propulsive and non-propulsive power storages need to have an integer number of cells. The initial number of cells from Equation (20) does not take into account the fact that a battery not only needs to provide energy, but also the correct voltage. The number of cells in series is determined by the required voltage for the electric motors, and can be found by rewriting Equation (18) into Equation (22).