This work presents a review of five different methods to determine the lumped series resistance R-S of solar cells and an experimental investigation of these to find the most reliable and robust method(s) for cell characterization under operating conditions. The methods under consideration are: fitting of the two-diode equation function to a dark IV-curve, comparison of a one-sun with a dark IV-curve, comparison of a Suns-V-OC with a one-sun IV-curve, comparison of two or more IV-curves measured at different illumination intensities, and computation of the area under a one-sun IV-curve. Firstly, for a quantitative evaluation, all series resistance values were plotted against the fill factor FF of the corresponding cell. The accuracy of the methods is quantified using a wide range of solar cells. Secondly, the robustness of the methods in the presence of other FF-limitations such as shunts is also explored. The results and the interpretation of a first analysis of small 2 x 2 cm(2) solar cells of the integration method led to a successful improvement of this method, which was proven by a second measurement. All the conducted investigations led us to the conclusion that of these five methods under consideration, the illumination intensity variation, the comparison of a Suns-V-OC with a one-sun IV-curve, and the modified comparison of a one-sun IV-curve with a dark IV-curve method are the most reliable and robust ways to determine the series resistance under operating conditions. (C) 2007 Elsevier B.V. All rights reserved.