Comparison of li-ion battery ageing models applied in photovoltaic stand-alone systems

Abstract

In this paper, three models to predict the ageing of li-ion batteries (LiFePO4/graphite) are compared. The models are applied in stand-alone (off-grid) photovoltaic (PV) - battery systems and PV - diesel (or gasoline) - battery systems. A high dependency on temperature is shown for all the models: the battery degrades at a higher rate as temperature increases. Also, all the models present a dependency on the charge and discharge rate: higher degradation occurs as charge and discharge rate increases. Also degradation is highly affected by pauses in between charge and discharge steps. Most of these dependencies are coupled and highly non-linear. In PV-battery systems the charge and discharge rate is usually low (lower than C/2), as the battery capacity is usually high (designed for several days of autonomy, usually 2-6 days). Even in PV-diesel or gasoline-battery systems this rate is low, although the battery capacity is much lower than in PV-battery systems. The models are evaluated in systems located near Zaragoza. In all the cases a 2.4 kWp PV generator is considered. The size of the battery bank and of the diesel or gasoline generator varies in the different cases. All the models show high battery life (>30 years) in cases of PV battery systems (with a battery bank of 4 days autonomy). However, in the cases of PV-gasoline-battery systems, there is a great difference in the estimation of the lifetime obtained by the different models.