Lithium-ion batteries are electrochemical storage devices that occupy an important place today in the field of renewable energy applications. However, challenging requirements of lithium-iron-phosphate LiFePO₄ (LFP) batteries in terms of performances, safety and lifetime must to be met for increase their integrations in these applications. It is important to identify the origins and symptoms of battery aging and to quantify the various aging modes. In this context, the aim of this paper is to develop a reliability approach which ensures the evaluation of LFP batteries aging using causal tree analysis and electrical equivalent circuit model. The causal tree enables a good understanding of common aging modes such as the loss of lithium inventory, loss of active mass, electrolyte degradation and corrosion of the current collectors and a comprehensive identification of their potential causes. The determination of electric equivalent model parameters is based on experimental voltage responses under different discharge rates and operating temperatures. This experimental study allows creating database parameters of a new battery, which takes into account the temperature effect using the fuzzy logic method. The proposed diagnostic system is based on the use of electrochemical impedance spectroscopy (EIS) technique to determine the parameters of used battery and compared them to the parameters of new battery. The comparison results allow quantifying the depth of the various aging modes as well as the number of remaining cycles. This study is completed by testing three LFP batteries that give insights into the feasibility and efficiency of the proposed diagnostic method.