Bi-linear capacity decay and internal resistance increase of lithium iron phosphate cell in electric -rickshaw application

Abstract

Several possible battery degradation modes for various batteśry chemistries were available in the literature. Eventhough standardized protocols can quantify capacity fade in controlled environments, the outcomes are not useful to provide sufficient knowledge of the degradation mechanism in commercial batteries for practical applications and the results are less informative to make any correlation to the degree of fading with underlying mechanism. Lithium Ferrous Phosphate (LFP) cell is entering into e-rickshaw segment due to low life of lead-acid battery. The e-rickshaw driving profile is unique due to its frequent start and stop operations. In the present work LFP cell is subjected to a newly developed life cycle test protocol which mimics the driving profile of e-rickshaw in real life condition. Cylindrical 3.2 V/ 6Ah batteries are discharged to various Depth of Discharge (DOD) in e-rickshaw driving profile. The results showed that the batteries could deliver 750–800 units Ah turnover which is independent of DOD. The capacity decay and resistance increase are bi-linear. In the first phase the capacity decay is due to growth of Solid Electrolyte Interphase (SEI) which consumes active lithium and in the second phase decay is due to isolation of active material in negative electrode. The implication of actual field conditions like a) higher ambient temperature in hot climate b) more than 75 % idle time and c) higher DOD on the battery is discussed with respect to life.