In the project Online Safety Assessment of Lithium-Ion Systems in Operation (OSLiB), safety-critical conditions for lithium-ion batteries are to be investigated from the cell to the system. The investigations start from specially manufactured cells with deviating production parameters, such as incomplete electrolyte wetting or faulty electrode positioning, in order to simulate aging-related effects in the cell that result in safety-relevant effects, such as lithium plating. In addition to the in-house manufactured cells, commercially available cells that serve as a reference will also be investigated. In a first step, the cell behavior in different operating phases (charging, pauses, cycling) and conditions (temperature, pressure) will be investigated. On the one hand, diagnostic procedures will be applied that rely on additional excitation and hardware for online use. On the other hand, online diagnostic methods will be investigated that are based only on “classical” battery management system data (voltage, current, temperature) and variables derived from them. The aim of the investigations at cell level is the diagnosis of safety-critical states, such as lithium plating, critical temperatures and the detection of fine/short circuits. It will also be investigated how safety-critical conditions can be easily replicated with commercial cells. In a second step, the effects of the safety-critical states will be scaled from the cell level to the system level and evaluated. Here, the localization of safety-critical cells in a parallel or serial network will be investigated by detecting inhomogeneity (temperature, current sharing, etc.) among the cells. Not only does the inhomogeneity itself matter, but also the rate at which it changes. The localization of the inhomogeneity should ultimately facilitate the targeted use of diagnostic methods that start at the cell level. The third step is to validate the diagnosed conditions. For this purpose, (post-mortem) analyses will be used to investigate the effects in the cells. Among other things, the amount and type of lithium plating will be determined. Finally, the necessary requirements for the battery system and the BMS are to be worked out in order to be able to apply the diagnostic procedures in operation. The focus is on the requirements for the measurement systems, the applicability to different system interconnections, the cell format and the cell chemistry, as well as the operating state and boundary conditions. The error susceptibility of the diagnostic methods will also be assessed. With the online detection methods for lithium plating, temperature anomalies, and fine shorts, the most common safety-critical events can be detected during operation. Thus, new cell concepts can be promptly investigated for their safety characteristics in system prototypes in operation and evaluated without time-consuming post-mortem examinations. This enables short-term feedback to cell development and rapid further development of novel cell designs. In addition, safety margins can be reduced due to improved online diagnostics. Thus, OSLiB is very close to the goal of the innovation pipeline – a technology that is ready for industry – and intends to illustrate this with the use of commercial cells and a BMS demonstrator. By using standard available data for diagnostics, few new system components are needed. The online capability means that safety-critical conditions can be detected early, avoiding premature system failures and consequent increased disposal costs.