High impedance short. If it was a dead short, battery would be arcing as soon as you put it in. Take an ohm reading from head center post+ to ouside head-. Not sure your driver but in off mode with twist head you should have open load. rotate the head and you should have a high impedence in the Megaohms. If you dont then one of your components in driver is bad. I apologize if this is brief but it can get entailed if its more complicated than this. There is probably only 3 possibilities 1. Mechanical switch failure 2. Led failure 3. Chip failure on board. All 3 can be checked with a DVM