The increased usage of absorbant glass mat (AGM) lead-acid starter batteries requires a broadening of the usual methods for battery analysis. The determination of the acid filling level has developed to a standard technique of quality control for AGM batteries. However, the distribution of acid-filling levels over the battery cells can also be used as a criterion for the evaluation of ageing mechanisms. Acid-filling level of starter batteries that have been aged in the laboratory and on the road were determined and hypotheses to correlate the results with different use cases were shown.