  Conduction in bulk polycrystalline high-T$_c$ superconductors with relatively
high critical currents has been shown to be percolative. This phenomenon is due
to weak links at grain boundaries. These weak links are the major limiting
factor for technological applications which require high current densities. We
formulate a model of these materials which can be reduced to a nonlinear
resistor network. The model is solved by analytical approximations and a new
numerical technique. The numerical technique is variational, which makes it
capable of solving a wide variety of nonlinear problems. The results show that
an unambiguous connection can be made between the I-V characteristics at the
microscopic level and the macroscopic electrical properties.
