In motorbikes the role of the alternator is very important to supply electric current to batteries which will be distributed to electrical components such as lights, starter dynamo, CDI (Capacitor Discarger Ignition) and other electrical system components. To increase the electric current produced, it is necessary to modify the halfwave alternator to be fullwave by changing the coil scheme on the alternator. The method used in this study is to test the standard alternator / halfwave that has been modified into a fullwave alternator on the Honda Supra X 125 D. The tests are current, voltage, torque, and power. The maximum current produced by a standard alternator / halfwave is 3.7 A (Ampere). After a large fullwave modification, the maximum current has increased by 46.8% to 7.9 A (Ampere). The result of modification of the halfwave alternator to fullwave alternator has no significant effect on torque and power. Under standard motor conditions using a standard alternator, maximum torque of 10.48 Nm at 5637 rpm. After being modified, the maximum torque that can be achieved is 10.71 N.m at 5688 rpm. In the condition of a standard engine motorbike using a standard alternator / half-power, the maximum power that can be achieved is 9.4 HP at 7142 rpm engine speed. After being modified, the maximum power that can be achieved is 9.7 HP at 6966 rpm engine speed.