Before you do that, make sure the headlight is working. In the headlight there are two bulbs, one standard low beam / high beam headlight bulb and one running light bulb. It is easy to mistake the running light bulb being on for the headlight working. If the headlight bulb isn't working the regulator won't properly charge the battery. Because it uses the voltage on the headlight circuit after the headlight bulb to measure battery voltage. The bulb can be burnt out or the circuit might be off (blown fuse or faulty relay).
You may need a new regulator, but make sure before going that way.