The thermal conditions of lithium-ion cells significantly influence performance, safety and aging of the battery system. In the research group ELS the thermal state of single cells and cell modules is investigated experimentally and by thermal 3D simulation. The investigations consider heat generation and thermal behavior under different load scenarios. Current research focuses on applications such as Fast Charging, as especially high currents are thermally demanding for battery systems. In addition to the consideration of the maximum temperatures at cell level, temperature inhomogenities at module level are investigated, which can lead to undesirable aging effects over lifetime. The final goal of the research group is to develop innovative concepts for operating battery cells at optimum temperature level.