A secondary battery includes a battery case configured subject to the configuration of a conventional 9V or 1.5V battery and a battery body and a battery charger mounted in the battery case. The battery charger controls the battery body for charging and voltage output, and provides a USB socket as charging interface and positive and negative electrodes as discharging interface. Thus, the secondary battery is connectable to a USB plug of a cell phone battery charger or computer for charging, and can be installed in an electric product like a conventional battery cell to provide DC power to the electric product. Under the provision of current detection function and voltage adjusting function, the 1.5V secondary battery can be connected in series or in parallel with one or a number of micro resistor-provided virtual batteries to output a voltage subject to its linking arrangement.