We studied Li/Li4Ti5O12 and Li/LiCoO2 half-cells as well as Li4Ti5O12/LiCoO2 coin-type battery cells at different state-of-charge over several cycles by means of electrochemical impedance spectroscopy. A simple equivalent-circuit model has been applied to probe changes at the internal interfaces of these batteries. The parameters of this model are used to follow the state-of-charge and state-of-health of the cells. Moreover plain empirical relations were introduced to accurately delineate those courses. (C) 2012 Elsevier B.V. All rights reserved.