Methods for calculating the state of charge of vanadium redox flow batteries: an analysis of the relationships

In the future, energy storage systems are expected to play a key role in the transition to low-carbon energy systems. Increasing renewable energy deployment rates require the integration of batteries that ensure the stability and security of the power system and mitigate the unstable behavior of renewables. Efficient operation of the batteries themselves is also important: it extends their life, reducing operating costs. Monitoring of the state of charge is one of the key tasks to help battery operation. This paper studies the existing explicit methods for calculation of the state of charge of vanadium flow batteries: the open-circuit voltage-based method and the Coulomb counter method. The relationship between them and the possibility of using them together to achieve more reliable and accurate charge state monitoring is investigated. In contrast to existing works, we derive an analytical expression for the overall state of charge that takes into account both main components of the battery, namely its stack and tanks. Analyzing their contributions reveals some shortcomings of existing approaches widely used to calculate and monitor the state of charge of flow batteries.