Differential voltage (dV/dQ) curve is examined to analyze the degradation of 30 Ah commercial lithium-ion batteries consisting of a Mn-based cathode and graphite anode during discharge. It is observed that the dV/dQ peak became sharper after the capacity faded. The corresponding peak change is confirmed by analysis of the electrode potential separation analysis using pseudo reference electrode and attributed to graphite voltage step at LiC12 single phase. In addition, the change in peak shape is not related to the degradation of lithium distribution in the electrode but is strongly related to the intercalation content in the graphite anode. Because the change in peak shape is observed only during discharge, charge voltage is recommended for the non-destructive analysis of lithium-ion batteries with graphite anode.