Here, to investigate the impact of battery operation on crossover, we develop a new three-pump configuration for on-line solution-state 1 H NMR spectroscopy (Figure ). In previous in situ spectroscopic redox-flow battery studies, a two-pump configuration was used meaning that the flow rate supplying the NMR and redox-flow battery were kept the same. The flowrate needed for quantitative NMR is an order of magnitude lower than the flowrate needed for a functional redox-flow battery. The flow rate affects both battery electrochemistry and crossover rates, so it is important to study the battery under real operating conditions. Here, a third pump is therefore used to decouple the NMR and AORFB flow, enabling quantitative 1 H NMR detection without compromising the battery operating conditions (see Methods S1-9, Figure and Figure ). For this study we focus on the 2,6-dihydroxyanthraquinone (2,6-DHAQ)/ferrocyanide battery (Scheme 1 and Figure ) equipped with a Nafion membrane, as it is often used as a benchmark model system. We find that battery operation has a significant impact on 2,6-DHAQ crossover. In particular, we observe that the 2,6-DHAQ crossover rate increases with the current applied during constant-current charging steps, revealing that battery operation is an important factor to consider in any crossover-mitigation strategies. Scheme 1. Half-reactions for i) the anolyte and ii) catholyte.