The measured heat flow of graphite/NMC lithium ion cells under charging conditions show a characteristic and easily identifiable signal at the onset of lithium plating on the graphite electrode: A marked decrease in heat flow signals the full lithiation of the graphite host. The origin of this signal is shown to arise from the combined effects of entropy and cell over potentials. This signal allows for an accurate measure Of the maximum amount of lithium intercalation possible in the host. Metallic lithium deposition begins within 5-7 mAh/g after the heat flow begins to decrease. Two different types of graphite were examined; G25 and MCMB. The onset of lithium plating was detected at 336 mAh/g for the G25 graphite and 297 mAh/g for the MCMB graphite, yielding empirical formulas of Li0.888C6 and Li0.804C6, respectively. The effect of plated lithium on the electrode/electrolyte reactivity was also examined by precise measurement of the coulombic efficiency, parasitic thermal energy and cell capacity fade. These measurements then allowed for the calculation of the efficiency of lithium plating on the graphite surface: 0.98 and 0.97 for 025 and MCMB graphites, respectively. (C) 2013 The Electrochemical Society. [DOI: 10.1149/2.049304jes] All rights reserved.