This dataset contains time series data (time, power, current, voltage, temperature, grid frequency, SOC, status) of the field operation of a stationary large-scale battery storage system in the German power grid (frequency containment reserve and 15 min intraday trading). The dataset originates from the operation of the M5BAT (Modular Multi-Megawatt Multi-Technology Medium Voltage - Battery Storage System) battery storage system at RWTH Aachen University and is divided into 11 partial datasets. One partial dataset is included for each of the 10 battery units of the M5BAT large-scale battery storage system. One partial dataset is included for the entire system, which can be seen as a measurement at the grid connection point at 10 kV medium voltage. The file M5BAT_04-2023_RAW.zip contains the individual *.csv files with the time series data. The file Report_04-2023.pdf contains a description of the data, an evaluation of the data and the license.