Further variation within each of the two groups of values is a result of differences in determining the voltage drop from the GCD data. Assigning the voltage drop is highly subjective and varies from researcher to researcher. To eliminate this variation going forwards, we recommend the agreement and application of a consistent criterion for measuring the voltage drop from GCD cycles, as outlined in previous work . For this study, the internal resistance was calculated at a single, fixed applied current. Measuring the voltage drop across a series of GCD experiments with different applied currents would give a more reliable resistance calculation.