## 4 Results and Discussion
It has also been noticed that the higher the temperature during the measurement, the higher the results of the direct-energy-loss method and the current step method converge. Comparing the graphs from the measurement at 2 C and at 3 C (Figure 9), it is visible that the direct-energy-loss methods provide almost identical results within the measurement accuracy for temperatures from 20 \({}^{\circ}\)C to 40 \({}^{\circ}\)C as it is visible for the LMO cell. The point that the calorimeter method provides higher results in absolute numbers than the other methods can be addressed here as well. Figure 10 shows the internal resistance of the LTO cell at various temperatures. Due to the small current during the measurement, Figure 10 (LTO cell at 1 C) has some significant measurement uncertainties. The current at 1 C is 1.3 A, and the resulting heating of the cell is very low. This increases the influence of the accuracy of the temperature measurement. The measurements with 2 C are shown in Figure 10 (LTO cell at 2 C). At 0 \({}^{\circ}\)C, the calorimeter method and the direct-energy-loss method are within the same measurement accuracy. The current step method is significantly lower, around 40% less than the calorimeter method. With increasing temperature, the internal resistance decreases due to the temperature dependence of the cell. This is visible for all methods. The current step method consistently yields the lowest results compared to the other methods. The results from the calorimeter method for the LTO cells are between 3 m\(\Omega\) and 5 m\(\Omega\) higher than those of the direct-energy-loss method and the results of the current step as it is visible in Figure 10 (LTO cell at 2 C). The value of the internal resistance of the LTO cell differs from the other cell chemistries, as it is significantly lower. At 3 C, the graph shows the same behavior as at 2 C, with the calorimeter method providing the highest value for internal resistance and the current step method providing the smallest values for each measured temperature. It is noticeable that all the measurements have a wide range of internal resistance values, which are significantly different for each used method. The values obtained at 2 C and 3 C for the calorimeter method are always higher than the values (covering both parameter resistance and temperature) of the remaining two methods for each cell chemistry. When looking at the results obtained for the cells, it is noticeable that the calorimeter method is between 10% and 20% higher as compared to the direct-energy-loss method. A possible explanation for such behavior of the calorimeter method is due to additional heating within the calorimeter. This can be caused by contact resistance at the cable connections and the resistances of the cables themselves. Figure 10: Measurement results for the internal resistance of the LTO cell at 1 C, 2 C, and 3 C from temperature 0 \({}^{\circ}\)C to 40 \({}^{\circ}\)C.  
Comparing the cells with each other is rather difficult because of the different currents, per the c-rate. For LMO and NMC cells, which have very similar specifications, the direct-energy method and the current step method show nearly identical results at temperatures from 20 \({}^{\circ}\)C to 40 \({}^{\circ}\)C and at c-rates from 2 C to 3 C. These are the cells with the highest current per c-rate due to their higher capacity. The heat generation inside the cell is important for the calorimeter method, as small heating leads to high measurement uncertainties. When using the calorimeter method, it must be ensured that the measurement current is set high enough. No impact on temperature has been identified due to the methods used. Some generic observations made is that the temperature difference between the energy-loss method and the current step method is higher at low temperatures (between 0 \({}^{\circ}\)C and 10 \({}^{\circ}\)C) than at temperatures above 20 \({}^{\circ}\)C.