Microporous polymer membranes based on poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) with barium titanate (BaTiO3) ceramic nanoparticles at different concentration (4, 16 and 32%) and average sizes (10, 100 and 500nm) were prepared by thermally induced phase separation. The effects of fillers concentration and size on morphological, thermal, mechanical and electrochemical properties were evaluated. Pores remain homogeneously distributed after the addition of the fillers. In turn, the average pore size increases from ∼25μm for the pristine polymer to 90μm for the membrane with 32 wt% of BaTiO3. The porosity, on the other hand, remains nearly constant at ∼72%. The ionic conductivity increases in all temperature range with the addition of BaTiO3, being this increase larger for samples prepared with larger diameter filler particles. The stable operation window is at least 6.0V for all membranes. It was concluded that morphological, mechanical and electrochemical properties of the BaTiO3 filled P(VDF-TrFE) membranes actually depend on filler size and concentration the most adequate membrane for battery separator applications being the BaTiO3/P(VDF-TrFE) membrane with 16 wt% of 500nm diameter ceramic fillers.