Charge iteration is an FEM procedure, devised by the authors for the computation of electrical fields in open boundary domains. In the procedure, a closed fictitious boundary is introduced enclosing all the conductors. On this boundary the electrical potential is first guessed and then iteratively improved according to the current solution by means of the free-space Green's function until convergence takes place. In this paper the procedure is generalized by allowing not only Dirichlet conditions, but also Neumann and Robin ones to he imposed on the fictitious boundary. Some reduction in computational effort comes from this generalization as pointed out by theoretical and numerical examples.