## 5 Conclusions  
Three methods (the current step method without charge change, the direct-energy-loss method, and the calorimeter method) were implemented in this research to measure the internal resistance of four different lithium-ion cell chemistries. The cell chemistries used were LMO, LFP, NMC, and LTO.  
The calorimeter method yields, consistently, the highest value for internal resistance in absolute values presumably caused by additional heating at contact resistances from the cell's connection.  
For the current step method, the most optimal results obtained are at temperatures of 20 \({}^{\circ}\)C and above at C-rates from 2 C. Here, the current step method delivered similar results for the LMO and NMC cells compared to the direct energy loss method. For LFP and LTO cells, the current step method is around 15% to 25% below the energy-loss methods. However, below 20 \({}^{\circ}\)C, the deviation of the current step method is even higher. A possible explanation cannot be derived from such results.  
The utmost measurement accuracy is achieved at high C-rates (3 C). The calorimeter method showed a large measuring error. The reason for this is the low heating of the cells at small currents. In addition to the current and voltage measurement, the accuracyof the temperature measurement inflects the accuracy of the calorimeter method. The direct-energy-loss method and the current step method provide a similar accuracy of about \(\pm\) 1 m\(\Omega\) from 2 C, as they only depend on the accuracy of the current and voltage measurement of the battery tester.  
Overall, this experimental research presents the three internal resistance measurement methods and compares them against each other based on four different cell chemistries, three different C-ratings, and with a temperature range of 0 \({}^{\circ}\)C to 40 \({}^{\circ}\)C. According to the authors' knowledge, there is no such in-depth comparative analysis available in the literature. Therefore, this research widens the opportunity for future researchers to select the appropriate method for identifying the internal resistance of the cell.  
For future work to compare, especially, the calorimetric measurements to the other two methods, additional factors inside the calorimeter such as contact resistances must be considered. Therefore, the measurements can be repeated with an improved calorimeter, and the contact resistances can be measured and included in the evaluation. Additionally, the latest version of the current step method can be adopted from the literature for future experiments. Lastly, there is a possibility to change the approach up to a certain extent. Rather than taking the fixed C-rates for all types of cells, the researchers can use the equal value of current during experiments to ensure that the appropriate influence of the current on cell heating.  
Conceptualization--S.M.P. and H.-G.S.; methodology--S.M.P. and H.-G.S.; formal analysis--S.M.P., [PERSON] and Y.K.; investigation--[PERSON], [PERSON] and Y.K.; data curation--[PERSON] and [PERSON]; writing--[PERSON], Y.K. and [PERSON], supervision--[PERSON]; funding acquisition--[PERSON]. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to legal reasons.  
The authors declare no conflicts of interest.