The construction of reinforcements of the electricity grid cannot keep up with the increasing demand for transport capacity caused by the energy transition. This leads to increasing congestion of the grid. Therefore, the feasibility of battery energy storage systems as a solution is researched in this thesis. To make this a financially viable option, the free capacity of the battery energy storage systems is used to participate in the FCR and passive balancing market. Four (mixed integer) linear programming models are proposed to optimise the battery sizing and market participation. The feasibility of the solution is investigated through three case studies, which are based on three nodes of the distribution grid where structural congestion is expected. The study shows that for two of the three case studies a profitable business case can be theoretically accomplished.