Lead-acid batteries are the main technology used in renewable energy systems (RESs) and autonomous power-supply systems due to their maturity and low cost, factors that will remain valid for the next few years. It is often stated, however, that batteries in RES applications exhibit shorter lifetimes than those expected by manufacturers' data or those experienced in real traditional applications. Overall, in relation to all other components in RESs, the battery lifetime is quite short and has an intensive impact on the costs of the total system. The Fraunhofer-Institute for Solar Energy Systems ISE has developed a new generation of battery-management system (BMS), which improves the storage lifetime and reliability of batteries in RESs and thus reduces maintenance and lifetime costs considerably. The BMS allows new operating strategies not possible with conventional battery systems. For this purpose, the battery bank is split into several strings that are connected in parallel. Each string is individually switchable according to its current state-of-charge (SoC) and state-of-health (SoH), which are determined continuously. For each battery string, the BMS enables shorter cycles at low SoC, an increase in the battery current rate, and intensive full charges during normal operating conditions. Furthermore, the BMS considers the ageing mechanisms of each string and can operate with different battery types or battery technology. For this reason, the BMS combines the different advantages of each string and generates an optimized battery. Furthermore, the BMS is established as an active two-terminal network. System planners and designers can handle the BMS like a conventional battery, which implies no essential changes in electrical installations. (C) 2007 Elsevier B.V. All rights reserved.