A tertiary battery (TB) can be charged by heating or cooling via the difference in the electrochemical Seebeck coefficient α between the cathode and anode. Here, we investigated the battery resistance R and its effect on the performance of a laminate film-type Na1.48Co[Fe(CN)6]0.87 (Co-PBA)/Na1.76Ni[Fe(CN)6]0.94 (Ni-PBA) TB. We found that the charge-transfer resistance R ct and diffusion resistance R dif are the dominant components of R, while the solution resistance R s has a minor role. Regardless of the solute type, R varied inversely proportional to the Na+ concentration M. In a high-R TB, the thermal voltage V TB and discharge capacity Q TB per unit weight of the total active material are significantly suppressed, which is quantitatively explained in terms of the voltage drop (IR, where I is current) during the discharge process.