Disconnect charging lead from battery, connect small test lamp in series between lead and battery positive terminal. DO NOT START ENGINE. Test lamp should not light. If it does light, stator or rectifier is defective. Unplug rectifier. If test light goes out. The rectifier is shorted. If test light does not go out, stator is grounded. Stator should also be checked for continuity as follows: Use multimeter, set on resistance scale. Touch one test prod to positive lead. Touch other test prod to each of the other pins in the connector. Unless the meter shows continuity at each of the pins, the stator winding is open and the stator must be replaced.
If the rectifier is shorted it is creating a path to ground. Unplug the defective (shorted) rectifier and it will start from a charged battery. It just won't charge the battery.