A method of measuring and displaying the actual quantity of electricity of a rechargeable battery being charged by an external power source via a charger. The method includes issuing a power out signal from the charger to temporarily disable the power source; sending a signal indicating the actual quantity of electricity of the battery as feedback to the controller in response to the disabled power source; and configuring the controller to have a reference voltage so as to compare the reference voltage with a voltage of the feedback actual quantity of electricity of the rechargeable battery to obtain a voltage ratio, and issue a display signal to a display for displaying a correct measurement based on the voltage ratio.