## 3 Methodology
The cables for the cell and the calibration heater ran through the plug of the flask, as well as the temperature sensors (CTP-5000), as shown in Figure 3.  
Figure 4 provides an overview of a cell connection with its measurement devices. One channel of the battery tester can handle 6 A. To have sufficient current, two channels were connected in parallel. The calibration heater was connected to the third channel of a rack. Before the measurements, each new cell was cycled 10 times from charge cut-off voltage to discharge cut-off voltage with the Neware Battery Tester. After that, the cells were set to 50% SoC again with the Neware Battery Tester. \begin{table}
\begin{tabular}{l c c c c} \hline  & **HETER** & **Samsung SDI** & **LTO Battery Co.** \\  & **HTCT18650** & **INR18650-29E** & **LTO18650** \\ \hline Nominal capacity [mAh] & 3100 mAh & 1600 mAh & 2900 mAh & 1300 mAh \\ \hline Nominal voltage [V] & 3.7 V & 3.2 V & 3.6 V & 2.4 V \\ \hline Charge cut-off voltage [V] & 4.2 V & 3.7 V & 4.2 V & 2.8 V \\ \hline Discharge cut-off voltage [V] & 2.5 V & 2.5 V & 2.5 V & 1.5 V \\ \hline Discharge current [A] & 10 A & 4.80 A & 8.25 A & 13 A \\ \hline Temperature range (charging) [\({}^{\circ}\)C] & 0 \({}^{\circ}\)C to 45 \({}^{\circ}\)C & 0 \({}^{\circ}\)C to 45 \({}^{\circ}\)C & 0 \({}^{\circ}\)C to 45 \({}^{\circ}\)C & 0 \({}^{\circ}\)C to 45 \({}^{\circ}\)C \\ \hline \end{tabular}
\end{table} 
Table 1: Cell specifications. Measuring the internal resistance of four different cells started with placing the four calorimeters inside the climate chamber a day before the measurement. The internal resistance is strongly temperature-dependent. To measure it at a certain temperature, the heating through the calibration and the heating through the measurement cycle must be considered. The target temperature (measurement temperature) has been selected so that it is in the middle of the temperature rise that is passed through during the measurement cycle. Previous measurements showed that the calibration of the system with the calibration heater leads to a temperature increase of about 1 K inside the calorimeter. This was considered when tempering the calorimeter liquid as well. The climate chamber was set to the same temperature as the calorimeter liquid to minimize temperature exchange between the calorimeter and the environment. When the liquid reached the desired temperature inside the thermostat, it was filled into calorimeters with a syringe. The climate chamber was then closed, and the temperature inside the calorimeters was recorded. Once the temperature was stable, measurements were undertaken. Figure 4: Circuit diagram of a measurement setup. Figure 3: Structure of the measurement setup. The measurements started with a first resting period of 5400 s (90 min). This was necessary to determine the temperature differences between the calibration and measurement cycle. For calibration, the calibration heater was powered with 5 V for 420 s to bring a defined amount of energy into the system, followed by the second resting period of 5400 s (90 min). Thereafter, the measurement cycle was applied to the cells and the time was recorded by the Neware battery tester with a resolution of 1 Hz. After the measurement cycle, there was a final resting period of 5400 s (90 min). Figure 5 provides an overview of the whole measurement procedure. The constant-defined measurement cycle (as shown in Figure 5) was used for all three methods: the current step method, the direct-energy-loss method, and the calorimeter method. It was noticed that continuous charging and discharging would exceed the safety limit of the cells, especially with high C-rates. Therefore, the cycles were defined with shorter charge/discharge steps. As can be seen in Figure 5, the cycle was set for the 40 s that contains charging, discharging, and rest periods. Then, this was repeated 45 times, which equates to 30 min in total for each cell.