The Fe/V redox flow battery has demonstrated promising performance with distinct advantages over other redox flow battery systems. Due to the less oxidative nature of the Fe(III) species, hydrocarbon-based ion exchange membranes or separators can be used. Daramic® microporous polyethylene separators were tested on Fe/V flow cells using sulphuric/chloric mixed acid-supporting electrolytes. Among them, separator C exhibited good flow cell cycling performance with satisfactory repeatability over a broad temperature range of 5–50 °C. Energy efficiency (EE) of C remains around 70% at current densities of 50–80 mA cm−2 in temperatures ranging from room temperature to 50 °C. The capacity decay problem could be circumvented through hydraulic pressure balancing by means of applying different pump rates to the positive and negative electrolytes. Stable capacity and energy were obtained over 20 cycles at room temperature and 40 °C. These results show that extremely low-cost separators ($1–20 m−2) are applicable in the Fe/V flow battery system with acceptable energy efficiency. This represents a remarkable breakthrough: a significant reduction of the capital cost of the Fe/V flow battery system, which could further its market penetration in grid stabilization and renewable integration.