We develop an ultrahigh-precision charge/discharge system (UHP C/D) that can control and measure charge and discharge currents precisely. By using our UHP C/D, we observe that the coulombic efficiency of graphite composite anodes depends strongly on density and loading. From the relationship between the irreversible capacity and loading calculated by the linear regression method, two kinds of irreversible capacity are identified; one is proportional to composite loading, while the other is loading-independent. This result suggests different side reaction rates of the composite interior and surface during the charge/discharge process.