This paper gives an overview of the components and failure modes that should be considered when studying the reliability of grid-size Battery Energy Storage System (BESS). Next to failures of the primary component, a reliability study should consider the failure of the protection, failure of the communication, and failure of the control system. After all the diagnosed failures, qualitative fault tree analysis (FTA) based on Boolean logic is introduced as a basis of further analysis. Storage installations have a number of applications, each of which requires a different look at the reliability. The following applications are discussed in the paper: improving supply reliability, compensation power flows, market participation and frequency control.