This document provides guidance for an objective evaluation of flow batteries by a potential user for any stationary application. This document is to be used in conjunction with IEEE Std 1679, IEEE Recommended Practice for the Characterization and Evaluation of Emerging Energy Storage Technologies in Stationary Applications. A flow battery is characterized by electrolytes flowing past both electrodes. Examples include: - Redox flow batteries, such as vanadium redox - Hybrid flow batteries, such as zinc-bromine The outline of IEEE Std 1679 is followed in this document, with tutorial information specific to flow batteries provided as appropriate. Examples of tutorial information include technology descriptions, operating parameters, failure modes, safety information, battery architecture, and qualification and application considerations. This document does not cover sizing, installation, maintenance and testing techniques, except insofar as they may influence the evaluation of a flow battery for its intended application.