Battery energy storage systems are a key enabling component for the transition from passive to active distribution grids. Therefore, the number of battery energy storage systems is expected to increase exponentially in the near future. As such systems consist of grid-side power electronic frontends, this increase drives the need for accurate harmonic models that can be used in power system studies to analyze the future impact on harmonic levels and harmonic stability. The aim of this study is to develop a measurement based black-box model of a single-phase commercial battery energy storage system in frequency domain. A comparison is made between harmonic characteristics in charging and discharging mode, which evaluates the requirement for different models for the two operating modes.