1. A method for executing battery auto-learning applied to a portable data processor with a rechargeable battery and a current gauge comprising steps of: (a) charging said rechargeable battery until said rechargeable battery approaches a first saturation state; (b) resetting said current gauge when said portable data processor detects a first saturation state of said rechargeable battery; (c) discharging said rechargeable battery and counting the electric capacity discharged from said rechargeable battery by said current gauge; (d) stopping discharging said rechargeable battery and memorizing a count value counted by said current gauge when said portable data processor detects a specific discharge state of said rechargeable battery; (e) recharging said rechargeable battery to a second saturation state of said rechargeable battery; and (f) calculating a new total capacity of said rechargeable battery according to said count value counted by said current gauge and said specific discharge state.