Effective conductivity and mechanical properties of composite polymer electrolytes, in which the reinforcement phase is a sintered packed bed of Li-ion conductive ceramics particles, were estimated using finite element analyses. The computations targeted estimation of the effect of sintering degree, i.e. size of the inter-particle connective necks, on the overall properties of the composite. Methods for microstructure generation and computational procedures were presented. The mechanical ability of the membrane to block lithium dendrites was assessed based on a stability criterion, which depends on the computed effective stiffness. It was found that the minimum size of the inter-particle connections necessary to provide mechanical stability without losing the enhancement in conductivity was 0.05 times the mean particle radius.