A model-based method is proposed for the impedance-spectra analysis of valve-regulated lead-acid batteries. The electrochemical and double-layer processes are analyzed in a wide range of frequencies based on current, voltage, and temperature measurements. Both slow and fast dynamics of the positive and negative electrodes are evaluated. The method is shown to be accurate, enabling the detection of differences in capacity or other parameters even in slightly differing batteries. An exact relation between battery impedance and the electrochemical cell model is established giving useful information on the contribution of different elements and processes on the impedance. The proposed method is computationally effective due to the special analytical formulation developed for impedance analysis. In addition to the basic processes, the method also allows analysis of the fast-rate discharge and overcharge processes. (C) 2004 The Electrochemical Society.