A method includes providing battery cells for secondary batteries. The battery cells are charged to a fixed voltage (Vc). The battery cells are laid aside in an open circuit for a preset self-discharge time period (t). A voltage (Vt) is measured after the period t for each battery cell. A voltage difference (ΔV) is determined for each battery cell, wherein ΔV=Vc−Vt. A self-discharge rate (ρ) is measured for each ΔV. A self-discharge current (Ic) is calculated during the period t, wherein Ic is a function of ρ. The Ic is curve-fitted to the ΔV for each battery cell to obtain an Ic−ΔV equation. The Ic is divided into h number of grades. The Ic−ΔV equation is utilized to determine h number of grades of ΔV associated with each grade of Ic. Battery cells are selected having a same grade of Ic and ΔV for matching.