Internal resistance of cells of lithium battery modules with FreedomCAR model

Abstract

Internal resistance is usually calculated by EIS (Electrochemical Impedance Spectroscopy) method, which gives unrealistic low internal resistance values. In this paper internal resistance will be calculated from the voltage drop with FreedomCAR method where the validation of the results is much better (99%) than EIS method[1][12]. Batteries are often tested per cell. But in most cases more than one single cell is needed for an application and the characteristics of a module of cells is not the same. In other cases the whole module is examined as one big cell, without looking on the individual cells. But the weakest cell affects the performance of the whole module. This research goes deeper than the module approach on batteries: the behavior of individual cells is examined while they are working together in a module. The battery model consists in most researches of an ideal voltage source and a simple internal resistance[2]. In this work the advanced FreedomCAR battery model, created by Idaho National Laboratories (USA), is used: the cell is represented by an ideal voltage source with two internal resistances and two capacitors. Usually batteries are tested with very low constant currents (till 5% of the nominal current value) to show a high capacity value to the customer, while the customer needs the characteristics of the battery in real conditions. Here the parameters are calculated by testing the battery packets in high pulse conditions. The matching between the predicted and the measured voltage is proportional with the quality of the model. This was 99% (+-0.9%) in the tests. This means that the model is very close to the reality. Three types of Lithium-ion battery packets with 6-7 cells were tested.