Unwanted lithium-metal deposition on the negative electrode of a lithium-ion cell causes capacity loss due to poor lithium deposition and stripping efficiency and the possibility for internal short circuits. Internal short circuits may cause thermal runaway, which is especially dangerous in applications requiring many individual cells. This article proposes a method capable of identifying the onset of unwanted lithium deposition in-situ using cycles to full depth of discharge in a cell of any form-factor. The two most important factors affecting the average voltage of a Li-ion cell under load are the internal resistance increase and loss of lithium inventory. Increasing internal resistance increases average charge voltage and decreases average discharge voltage. Loss of lithium inventory, which occurs rapidly during unwanted lithium deposition and stripping, increases both average charge and average discharge voltage. Increasing internal resistance and loss of lithium inventory have a linearly additive effect on average voltage; therefore, tracking the average of average charge- and average discharge-voltages versus cycle count allows one to determine where rapid changes to lithium inventory onset, indicative of the onset of unwanted lithium deposition. (C) The Author(s) 2018. Published by ECS.