We designed an acoustic monitoring system to detect hydrogen bubbles in the analyte solution of all-vanadium redox flow batteries (AVFBs). The system has a specially designed ultrasonic probing cell made of borosilicate glass that can be integrated into the flow system on the analyte side (Figure 1). The analyte solution firstly flows through the negative electrode, where the chemical reaction occurs and hydrogen bubbles are generated, and then flows into the ultrasonic probing cell. The ultrasonic transducer transmits signals into the probing cell and collects echoes that propagate through the electrolyte solution in the probing cell. Details about how the signals are transmitted and received are included in Appendix A. The echoes are collected by the ultrasonic receiver and sent to the computer for data analysis. The sound speed and the acoustic attenuation coefficient are calculated from these echoes, with the detailed methods given in Appendix B and C. Bubbles that flow through the acoustic path are reflected as abnormal values in the measurements of the sound speed and the attenuation coefficient. This method not only can detect bubbles in the analyte solution, but also can estimate the bubble flow speed. The monitoring system can continuously monitor bubble amounts in negative electrolytes without interrupting the battery operation, and thus provides a real-time noninvasive surveillance of the health status of flow battery systems. Figure 1. Schematic of the in-situ battery health monitoring system for hydrogen detection in an all-vanadium redox flow battery.