In this paper, the NSGA-III algorithm is implemented in order to find optimal operation in a grid-connected microgrid with Distributed Energy Resources DER in terms of minimal power imports and power losses. The optimal location and sizing of elements in electric networks, such as distributed generators or energy storage systems, is typically a long-term planning task for distribution networks. However, in this paper, that problem is reformulated in order to find an optimal operational scheme (location, and the hourly State of Charge) for Mobile Battery Energy Storage Systems MBESS in a microgrid considering charge/discharge efficiencies, three initial States of Charge SOC, three capacities for BESS, PV generation, power import/export costs and demand profiles. Then, the NSGA-III algorithm is adapted to find solution fronts to the location and dispatch of energy within BESS units. The numeric results obtained have shown that efficiencies constraint strongly the operation of MBESS units in microgrids in terms of the cost of active power imports, while the analysis without efficiencies allowed a more flexible range of operation in both objectives. In comparison with the system without MBESS, reductions of up to 10% in power losses and up to 16% in power import costs have been found. Overall, optimal placement and operation of MBESS might bring further improvements in already optimized networks, and flexibility to the operation of microgrids by enhancing the use of distributed generators and relieving the network's load during peak demand.