The Doctoral Thesis analyses classical multilevel converter topologies for application in battery energy storage systems and proposes novel multilevel converter configurations are proposed in the Doctoral Thesis. Proposed multilevel converter with independent sources and unfolding circuit allows for reducing switch count and, therefore, reducing overall losses of the converter without losing the advantages of classical topology. A hybrid modulation approach is introduced in the Thesis, and the application of hybrid modulation of multilevel converter structures in DC-DC and DC-AC systems is described.