## 2 Experimental Setup  
Firstly, a database including nine Hacker Topfuel Eco-x batteries [7] of 10 lithium cells each, 5000 mAh of factory capacity, 3.7 V of nominal voltage, and 4.2 V of charge voltage per cell is created. These cells were used in the racing of aeromodeling aircrafts during the last five years, from 2016 to 2021. Some values are recorded on the cell level, others on a pack level. The data collected for each battery cell include the following:  
* Number of charging processes. * Dates of charging processes. * Initial voltage of each battery cell in the charging processes. * Final voltage of each battery cell in the charging processes. * The data collected for each battery pack include the following:
* Charge capacity stored during each charging process, measured in mAh. * Measurements of the internal resistance at the end of the charging processes. * Estimations of relaxation voltage drop. * Estimated ambient temperature in flight. * Differentiation between flight types between \"training flight\" and \"competition flight\". To obtain the data related to the charging processes, an intelligent charger, model \"icharger 4010 duo\" [8], was used (Figure 1). This charger can charge two batteries in parallel with a maximum power of 2000 W (max 1400 W per channel) and has some integrated options, such as the measurement of the voltage with a precision of 1 mV and the internal resistance with a precision of 0.1 \(\Omega\). All values can be saved on a micro SD card. Since the purpose of data collection was not initially to conduct a further academic study on them, the measurement conditions such as ambient temperature and rest times between use and charging were not predesigned, so full homogeneity cannot be expected. Still, this presents the kind of data that can be obtained for real-life applications. The charge profile used for these batteries was the same in all cycles performed, except for the initial voltage of each lithium cell, which depended on the previous use. A linear  
Figure 1: Icharger 4010 duo [8]. loading ramp was made at 3.8 A (0.8 C) until reaching 4.05 V; then the C-rate was halved, and another linear load ramp was made at 1.9 A up to 4.15 V; at that point, the charger automatically gradually reduced the current to 1/10 until about 4.17 V; finally, the battery finished charging at that final C-rate (0.19 A), reaching 4.2 V per cell. Therefore, the final voltage of each cell in all charging processes is 4.2 V (Figure 2). This charging profile was designed after multiple tests, looking to extend the life of the battery, but with an adequate charging time. The ambient room temperature and storage temperature were considered between 21 and 22 \({}^{\circ}\)C. The temperature of the battery during charging is estimated to have evolved from room temperature to approximately 41 \({}^{\circ}\)C maximum. These values are considered common, and the effects of possible variations were not considered. To estimate the ambient temperature during the discharge of the battery in flight, the official data of AEMET [9] are taken knowing the dates and the location where the corresponding training and competitions took place. As AEMET only offers maximum and minimum temperature data, a relevant rough estimate was made, taking into account the training timetables. Due to the high variability in aircraft flights, it is difficult to establish a clear differentiation between them. However, it was considered interesting to separate between training and competition flights, since in this case there is a clear difference in the rate of discharge. On average, approximately 65 A of average discharge current were estimated in training flights and 80 A in competition flights. Due to the particularity of real data collection, the resulting analyses cannot be considered to be purely scientific in nature. However, the use of real data can help to analyze the process itself and highlight main trends that contribute to battery aging, which can be connected to existing theory.