Due to the legal limitation of the CO2 vehicle emissions, different technologies were actual focused in the automotive research and development to achieve the requested targets and avoid financial penalties. The car manufacturer recognised the CO2 saving potential by a reduced vehicle weight. In addition, hybrid powertrain solutions, which increase the fuel efficiency by an intelligent combination of the internal combustion engine with the electric motor, were more and more introduced in the series production. Next to complex and cost intensive high voltage systems, hybrid approaches with a voltage level below 60 Volt were analysed as well. As additional voltage protection demands can be avoided, these installations are less complex. As a priority target, the increasing energy demand of the electrical on-board system should be covered and, in addition, the higher recuperation potential in comparison to a conventional 12 Volt system is used for a sustainable fuel reduction. The use of the electric machine as a traction motor and, therefore, to represent a hybrid vehicle with all common driving functions, is limited especially for series vehicles due the higher curb weight and the limited power of the low voltage system. This thesis deals with the combination of the prior mentioned technology trends and analyses in detail a low voltage hybrid approach in the context of an ultra-light vehicle concept (A-segment) with a curb weight below 700 kg. Belt-Starter-Generators with a voltage level of 12 Volt and 48 Volt were considered and compared to a powertrain configuration with a conventional alternator plus starter structure as this would be common in such vehicle class. Especially the powertrain architecture, which means the positioning of the Belt-Starter-Generator in the existing powertrain, is focused. Simulations as well as test bench measurements at the real powertrain investigate the different topologies regarding fuel efficiency and comfort criteria. Despite the 48 Volt level, an intelligent adaption of the Belt-Starter-Generator in combination with an established hybrid operating strategy generates hybrid functions as they are common for Full-Hybrid vehicles and lead to a fuel reduction up to -23,2 % (NEDC) and -16,1 % (WLTP) compared to a conventional 12 Volt configuration.