Influence of current and temperature variation on a LiFePO4 battery total capacity

Due to the dynamic conditions during the driving operation with rarely deep discharging, the on-board evaluation of the total capacity of the battery pack in a plug-in hybrid electric vehicle (PHEV) and electric vehicle (EV) is one of the most challenging tasks of a battery management system (BMS). In fact, the rapid dynamic variation of the current rate, the unsteady ambient temperature and stand stills of variable duration yield to a situation completely different in respect of the one met in the laboratory (i.e. constant continuous current discharge with a constant ambient temperature). The aim of this paper is to investigate the influence of current rate and temperature variation on the final total capacity that a lithium iron phosphate (LiFePO₄) battery is able to deliver. The effect on the total capacity of current and temperature is deeply investigated, both for a cell in a new and aged state: the current and the temperature have been changed during the discharge process continuously in a systematic manner, in order to prove if these factors influence not only the last part of the discharge process but also the early one. The execution of the same tests for cells in different aged state allows the comparison of the results and the identification of the factor influence variation with the battery lifetime. At the end, the repercussions that the current and temperature variation have in the online calculation of the actual total battery capacity are discussed, and a possible implementation for EVs and PHEVs on-board algorithm for capacity estimation is illustrated.