The performance of commercial separators at high charge rates was evaluated using Li4Ti5O12 and LiMn2O4 as negative and positive electrodes, respectively. Most of the porous separators tested induced a sharp decrease in the conductivity of the liquid electrolyte. The conductivity decrease was related to the amount of porosity, polymer/electrolyte affinity, and the size of the pores and their interconnection. The decrease in conductivity induced by the separator incorporation and the separator thickness seems to be relevant indicators for optimizing a separator dedicated to high charge rate lithium-ion batteries.