The use of active materials with high resistivity in lithium-ion batteries necessitates covering the surface of active particles with electron-conducting films. If this measure is insufficient, then carbon black is added to the electrode active layer. The ohmic losses are assessed by computer simulation of electrode’s active layers with active grains covered by a carbon film. Electrode’s active layer is modeled as a set of equal-sized cubic grains of the active material (covered with a conducting film) and the electrolyte; the grains are randomly distributed throughout the active layer. It is shown how the effective conductivity of the active layer decreases in this case. Furthermore, account is taken of the fact that carbon films represent a set of islets, which results in an additional decrease in the effective conductivity of the active layer. By computer simulations in combination with the percolation theory, it is found how the addition of carbon black can increase the conductivity of electrode’s active layer.