Accurate determination of the Coulomb Efficiency (CE) is essential for reliable assessment of battery aging. In this paper, we present a highly accurate measurement system developed at ZSW that minimises errors caused by mechanical, electromagnetic and thermal interferences. By implementing strict cycling protocols, optimised shunt design and zero offset compensation, our method provides more accurate CE values compared to conventional battery cyclers. Experimental validation on Samsung 35E 18650 cells under controlled conditions confirms that the proposed approach significantly reduces measurement error and improves lifetime prediction. This advance in measurement accuracy provides a robust tool for improved battery performance analysis.