Having observed operation-dependent 2,6-DHAQ crossover rates, we then examined how this related to each step of the battery charging protocol. In Figure ,b, the amount of 2,6-DHAQ in the catholyte is plotted along with the state-of-charge of the cell (i.e. the level of charge of the battery relative to its capacity). Accurately determining the state-of-charge of a redox-flow battery is an ongoing challenge in the field. Here we have measured this from the change in water (HOD) H chemical shift, which changes as a function of ferricyanide concentration and therefore stateof-charge (see Figure and Methods S7) and has previously been shown to be a reliable method. Using this approach, we found that both Cell 2 and Cell 3 reached >92% of their theoretical capacity (116 mA h) in the first 50 mA charge, highlighting the good performance of the battery.