In this paper, a state of charge (SoC) based characteristic diagram control concept for energy storage systems (ESS) within an industrial direct current (DC) microgrid is proposed. The inputs are the SoC of the ESS and the terminal voltage of the DC microgrid. The output is the charge and discharge current for the converter, which connects the ESS with the DC microgrid. An appropriate design concept for the characteristic diagram is then investigated to achieve a flexible control reacting on changing conditions within the DC microgrid. These could be a temporary overload, due to a changing number of grid participants or an additional feed-in through a photovoltaic (PV) system. The characteristic diagram design concept is even applicable without a deep knowledge of the load profile of the DC microgrid. The concept was analyzed and evaluated based on simulations with load profiles from robot cells. The results show that the SoC depends on the current load of the DC microgrid. If the load returns to average, the SoC of the ESS inclines back to the nominal SoC, which is pre-defined by the grid operator. Furthermore, in case of an appropriate design of the characteristic diagram, it can protect the ESS against overcharging or deep discharging.