A simple method is proposed for battery impedance characterization at low frequencies (relevant to the electrochemical processes) and at higher frequencies (relevant to the double-layer process). The electrochemical impedance at very low frequencies is described using discharge curve information. This method makes it possible to distinguish between outwardly similar batteries that differ in backup times. The double-layer impedance at higher frequencies is characterized using short testing pulses. It is shown that the double-layer impedance of the negative electrode is observable in hertz range while the positive electrode is observable at the boundary to the millihertz range. The double-layer impedance of deeply discharged battery is,observable at higher frequencies than the impedance of fully charged (or significantly discharged) battery because the electrodes are less overlapping in frequency and the spectrum shift to the higher frequencies is larger especially for the negative electrode. (c) 2005 Elsevier B.V. All rights reserved.