Rechargeable batteries serve as power sources for safety lighting and safety power supply systems. It is essential that they function reliably. If the power source is not available or only available to a limited extent, safety lighting or safety power supply systems cannot fulfill their tasks in an emergency. This endangers the safety of people in the building. This document gives an overview on the structure and possible operating states of a lead-acid battery, causes of typical battery failures and describes the schematic structure of a single-block monitoring system.