We consider the application of passivity theory to the problem of voltage stabilization in DC microgrids, which are composed of distributed generation units, dynamic RLC lines, and nonlinear ZIP (constant impedance, constant current, and constant power) loads. To this aim, we rst provide a novel result on stable interconnection of multiple passive systems and later consider its applications to microgrids. More specically, we consider decentralized multivariable PI controllers proposed in [27], and show that they passivate the generation units and the loads under certain conditions. To prove voltage stability in the closed-loop microgrid, we exploit properties of interconnection, passivity of individual components, and LaSalle's invariance theorem. Moreover, we provide explicit inequalities on control gains to design these stabilizing controllers. Control synthesis requires only the knowledge of local parameters and is always feasible allowing removal and addition of DGUs in a plug-n-play fashion. Theoretical results are backed up by simulations in PSCAD.