In situ state of health vanadium redox flow battery deterministic method in cycling operation for battery capacity monitoring

In this paper we propose a new method for monitoring of electrolyte's State of Health (SoH) in Vanadium Redox Flow Batteries. The keystone of our approach is a correlation between optical and electrochemical properties of electrolytes based on the shift of electrolytes reflective index (RI) values at the same open circuit voltage (OCV) and their relation to the SoH change. In addition, we propose a simple sensor for RI measurements that can be easily implemented as an in situ method for real-time monitoring. In order to calibrate the sensor, electrolyte SoH was determined using the capacity measured in the cycling battery operation reaching deep charge/discharge states achieved in a constant voltage technique. The derived correlation between RI and OCV provides a powerful technique for SoH monitoring without knowing the full history of the battery operation providing the least mean error of 1.83% at OCV of 1.4 V. As a result, the proposed method is an important step for development of advanced control-monitoring tools that could assure reliable and efficient long-cycling operation of VRFB systems.