Critical Assessment of The Explicit Methods For State of Charge Calculation In Vanadium Redox Flow Batteries

Energy storage systems are expected to play a key role in the transition towards low-carbon power systems. The increased pace of renewable energy deployment requires the integration of batteries ensuring stability and safety of the grid and smoothing the intermittent behavior of renewables. Efficient operation of batteries itself is also important. It prolongs the batteries lifetime reducing operational costs. One of the key technologies helping with battery management is the state of charge (SoC) monitoring.

The paper provides a critical analysis of the explicit methods for calculating SoC of Vanadium Redox Flow Batteries (VRFBs): the OCV-based method and the Coulomb counting method. We study the interrelation between them and how they can be used together to achieve more robust and accurate SoC monitoring. In contrast to existing works, we obtain an analytical expression for the overall SoC accounting for both main components of the battery, namely its stack and tanks. Our analysis of their contributions makes it possible to reveal some drawbacks of the existing approaches widely used for VRFB state of charge calculation and monitoring.