Abstract Galvanostatic cycling combined with a modified intermittent current interruption resistance determination method is presented as a fast, accurate, and comparatively simple analytical tool for following internal resistance changes in batteries over long‐term cycling. The technique is demonstrated here to study the influence of electrolyte composition and volume on the internal resistance of lithium−sulfur (Li−S) batteries. This approach is found to be particularly useful for the study of the Li−S system, where resistance changes considerably during charge and discharge as a result of compositional changes to the positive electrode and the electrolyte, but may also be valuable in the study of other battery systems.