Electrochemical energy storage has become important in the last years to improve the energetic efficiency of stationary and hybrid energy production systems. Lithium batteries represent the best choice, since they have high energy and power density, high number of cycles and a good temperature behaviour compared to other technologies. It becomes important the monitoring of the state of the battery to know online the energy content, to avoid dangerous operating conditions and to extend the useful life. In this work, we have defined a test procedure to study the ageing of LiPO (lithium polymer) batteries through the EIS (electrochemical impedance spectroscopy) technique. Through a fitting procedure performed on the acquired IS (impedance spectra), we have extracted the parameters of an equivalent circuit model to reproduce the voltage discharge curves of the batteries and to study the performance under load. Then we have introduced a relation between the ohmic resistance of the battery and the available capacity to evaluate the SOH (state of health) through the ToE (Theory of Evidence). The method is able to detect the SOH of the tested batteries with a maximum error of 3.73%, with an enhancement to 8.66% when an anomalous cell is considered. (C) 2015 Elsevier Ltd. All rights reserved.