Coulombic efficiency is a powerful metric for evaluating the performance of materials in rechargeable cells and batteries. The ideal Coulombic efficiency, the ratio of charge removed to charge inserted, is unity. Some specialized systems can accurately measure cell capacity and Coulombic efficiency within 0.001%, which requires precise control and measurement of current, voltage, time, and temperature. Most battery electrode and electrolyte research is not performed with such precise but complex systems. The purpose of this paper is to demonstrate a simple, robust procedure to measure and possibly improve the accuracy of capacity and Coulombic efficiency measurements on standard systems in their as-used state. This approach is built on a commercially available thin film rechargeable cell for micro or milliampere currents and can be extended to, e.g., 18 650, cells for higher currents. An improved method to display Coulombic efficiency data is also presented. Regular, consistent calibration of testing systems and reporting of system resolution at specified test conditions is encouraged.