Use an alternator and a battery. The alternator has a regulator to stop the 12V BATTERY from overcharging
Without any intelligence in the system you'd have several problems:
1. The excitation coil would drain the battery when the blades were not spinning.
2. The windmill would be difficult to start spinning because the regulator always fully excites the coil.
3. When the regulator cuts off charging it would result in the windmill spinning without resistance and potentially out of control.
To the OP, In general if you're trying to generate power below ~100W you should stick with solar.