## 4 Results and Discussion  
This section presents the results of each cell chemistry in Figures 6-9. The internal resistances determined by the three methods were each shown in the same color i.e., Blue--calorimeter method, Grey--current step method, and Orange--direct-energy-loss method. During the charging process of the LMO cell, the maximum cell voltage of 4.2 V was exceeded. Therefore, the measurements for 2 C at 10 \({}^{\circ}\)C and 3 C at 0 \({}^{\circ}\)C and 10 \({}^{\circ}\)C were not recorded. This could have been avoided with smaller C-rates. However, using smaller C-rates would not generate enough heat inside the cell to determine the internal resistance with the calorimeter method as mentioned in the previous section. For the LMO cells in Figure 7 (LMO cell at 1 C), the values of the calorimeter method and the direct energy loss method are almost the same at 112 m\(\Omega\), while the current step method value is 25% lower than the other two methods i.e., 85 m\(\Omega\) (Figure 7). The smallest measurement value of 29 m\(\Omega\) is recorded at the highest temperature (40 \({}^{\circ}\)C) for the current step and the direct-energy-loss method. For the LMO cell at 2 C and 3 C, the direct-energy-loss method and the current step method yield same results within the measurement accuracy. Consistently higher results can be seen for the calorimeter method. An explanation for this is additional heating inside the flask of the calorimeter through to contact resistances from the connection of the cells to the Neware Battery Tester. It is visible that there is a strong temperature dependence of the internal resistance. The measurement uncertainties depend on the current applied to the cells (higher current equals more heat equals less measurement uncertainties). Like LMO cells, LFP cells also exceeded the safety limit while charging. The maximum charging voltage of 3.65 V was reached during the measurement cycle's first charging step. Hence, the measurements for 0 \({}^{\circ}\)C at 2 C and 3 C and 10 \({}^{\circ}\)C at 3 C were not recorded. For the LFP cells in Figure 8, measurement errors for the calorimeter and direct-energy-loss methods were higher than those of the LMO cells in Figure 7. The reason is the smaller current of 1.6 A per c-rate, which leads to smaller heating of the cell compared, for example, to the LMO cells. However, it can be noticed that the current step method  
Figure 6: Example of area-compensation method from the measurements. The black curve represents the measured temperature inside the calorimeter. The red lines are the extrapolated resting phases necessary for the determination of the temperature differences. The blue lines are the determined temperature differences from the area-compensation method. presents consistently low internal resistance as compared to other values throughout the selected temperature range of 0 \({}^{\circ}\)C to 40 \({}^{\circ}\)C in absolute numbers. Figure 8 shows that the calorimeter method always provides the highest value for the internal resistance, followed by direct-energy-loss and current step methods for the complete range of temperatures. In contrast to the LMO cell, none of the methods provide matching results. Figure 8: Measurement results for the internal resistance of the LFP cell at 1 C, 2 C, and 3 C from temperature 0 \({}^{\circ}\)C to 40 \({}^{\circ}\)C.  
Figure 7: Measurement results for the internal resistance of the LMO cells at 1 C, 2 C, and 3 C from temperature 0 \({}^{\circ}\)C to 40 \({}^{\circ}\)C.  
Figure 9: Measurement results for the internal resistance of the NMC cell at 1 C, 2 C, and 3 C from temperature 0 \({}^{\circ}\)C to 40 \({}^{\circ}\)C.  
For the NMC cells, the measurements for 0 \({}^{\circ}\)C at 2 C and 3 C, along with 10 \({}^{\circ}\)C at 3 C, were not recorded as they exceeded the maximum cell voltage. Figure 9 (NMC cell at 1 C) shows that, at 0 \({}^{\circ}\)C, the results of the energy-loss methods are almost identical. The current step method is significantly lower.