Aging effects of twice line frequency ripple on lithium iron phosphate (LiFePO₄) batteries

The charge/discharge current profile is one of the most important factors that affects the behavior of lithium-ion batteries (LIBs). Most of previous studies evaluate the behavior of LIBs under pure constant current conditions, when in reality battery packs in arguably the most important applications experience alternating currents (AC), superimposed on DC components. So-called 'ripple currents' commonly present at twice line frequency (i.e., 100/120 Hz) and their presence forces designers of single-phase AC-DC converters to incorporate a large DC-link capacitor into these devices. And yet, all this effort presupposes that AC ripple constitutes a serious problem, when it is still unclear to what extent the LIB lifetime may be reduced by the presence of 100 Hz ripple current. While many studies claim that AC harmonics accelerate the degradation of LIBs, there are others that refute this. Different chemistries of LIBs have been included in these studies, however, a systematic study on cells with LiFePO₄ (LFP) as the cathode material, considered as one of the most promising chemistries in many different applications, has not been done yet. This paper presents the results of a detailed and systematic experimental study on aging impacts of 100 Hz-ripple currents on 13 fresh LFP cells. The degradation of the cells is characterized based on capacity fade and impedance rise. The results show that the superimposed 100 Hz ripple currents do not cause a significant amount of degradation on LFP cells. This means that in many inverters, the size of the DC-link capacitor is probably excessive, thereby allowing a marked reduction in size of this expensive system component.