Experimental investigation on the internal resistance of Lithium iron phosphate battery cells during calendar ageing

Lithium-ion batteries are increasingly considered for a wide area of applications because of their superior characteristics in comparisons to other energy storage technologies. However, at present, Lithium-ion batteries are expensive storage devices and consequently their ageing behavior must be known in order to estimate their economic viability in different application. The ageing behavior of Lithium-ion batteries is described by the fade of their discharge capacity and by the decrease of their power capability. The capability of a Lithium-ion battery to deliver or to absorb a certain power is directly related to its internal resistance. This work aims to investigate the dependency of the internal resistance of lithium-ion batteries on the storage temperature and on the storage time. For this purpose, accelerated ageing calendar lifetime tests were carried out over a period of one year. Based on the obtained laboratory results, an empirical ageing model was developed; the model is able to predict with accurately the increase of the internal resistance of Lithium-ion batteries during calendar (storage) ageing. Based on the proposed ageing model, it was found out that the internal resistance of the studied Lithium-ion battery cell will double after approximately eleven years if stored at 25°C.