The effects of current density, temperature, depth of discharge (DOD), and storage on the heat generation rate and faradaic efficiency (ɛ) of Li/TC and Li/SO2 cells have been determined. Several C-size commercial cells from different manufacturers havebeen tested. The faradaie efficiency for both systems was found to bevery high, typically 96-100% even at high current density and high temperatures (55°C). It does not changemuch with DOD and decreases only slightly with the increase of current density and temperature (tested up to 4.5 mA cm-2 at 50% DOD and 55°C). A performance degradation problem was found for some Li/TC cells. The heat factor, the ratio between the usefulelectric power and the thermal power generated by the cell, is about the same for fres Li/TC cells and Li/SO2 cells. However, some Li/TC cells stored for 3 years showed a poor heat factor. It was confirmed that the maximum thermoneutral voltage for the Li/TC and Li/SO2 cells is 3.80 and 3.22 V, respectively.