Undesired reactions in Li-ion batteries, which lead to capacity loss, can consume or produce charge at either the positive or negative electrode. For example, the formation and repair of the solid electrolyte interphase consumes Li+ and e(-) at the negative electrode. High purity electrolytes, elimination of water, various electrolyte additives, electrode coatings, and special electrode materials are known to improve cycle life and therefore must impact coulombic efficiency. Careful measurements of coulombic efficiency are needed to quantify the impact of trace impurities, additives, coatings, etc., in only a few charge-discharge cycles and in a relatively short time. The effects of cycle-induced and time-related capacity loss could be probed by using experiments carried out at different C-rates. In order to make an impact on Li-ion cells for automotive and energy storage applications, where thousands of charge-discharge cycles are required, coulombic efficiency must be measured on the order of 0.01%. In this paper, we describe an instrument designed to make high precision coulombic efficiency measurements and give examples of its use on commercial Li-ion cells and Li half-cells. High precision coulombic efficiency measurements can detect problems occurring in half-cells that do not lead to capacity loss, but would in full cells, and can measure the impact of electrolyte additives and electrode coatings.