An apparatus for determining a state of a rechargeable battery or of a battery has a sensor device and an evaluation device. The sensor device brings about an interaction between an optical signal and a part of the rechargeable battery or of the battery, which part indicates optically acquirable information about a state of the rechargeable battery or of the battery, and detects an optical signal caused by the interaction. The sensor device furthermore provides a detection signal having information about the detected optical signal. The evaluation device determines information about a state of the rechargeable battery or of the battery on the basis of the information of the detection signal. Furthermore, the evaluation device provides a state signal having the information about the determined state.