We show the first continuous in situ static Li-7 nuclear magnetic resonance (NMR) experiment on a plastic lithium/carbon cell. The electrochemical cycling was successfully performed inside the magnet. We particularly studied the insertion/extraction of lithium in a material designed to be used as negative electrode in a secondary battery; hard carbon fibers treated to reduce the irreversible capacity (x = 0.5). The reversible capacity is x = 1.5 (in LixC6), higher than with graphite for which the saturating composition corresponds to the first stage intercalation compound LiC6 (x = 1). Ex situ quantitative transmission electron microscopy gives a statistical description of the fibers; a core made of disorganized carbon coated by better organized pyrocarbon. Crossed with the values of capacity given by the current generator, in situ NMR indicates the chronology of the insertion. From the observed data, we propose that a Li-C metallic alloy is formed. By comparing the Knight shift with that of metal (Li-0), we estimate the charge transfer from Li to C. Amongst the two types of lithium found, the first might be assigned to Li intercalating (Li+0.66) into the smallest spaces between the graphene layers. The second represents most of the reversible lithium and is assigned to be quasimetallic (Li+0.1). Its longitudinal relaxation time T-1 is also near-metallic. The quality of this carbon comes from its ability to host such a species. (C) 2003 American Institute of Physics.