A simple zener (or two) is not really accurate enough. 
I would put one or more power transistors (maybe my old favourite the 2N3055) on the heatsink and add some extra bits to make something that behaves like a precision power zener. It's a standard circuit that you can find in any decent electronics book.
If you intend going to the trouble of adding variable pitch or self feathering blades why not adopt a simpler mechanical solution: a steerable tail fin. As the turbine speed increases you drive the tail fin off to one side and thus turn the thing out of the wind.
And finally --- I would always have some kind of meter to give me a quick view of the state of the batteries. I made one from an LM3914 bar-graph driver IC and a string of eight LEDs. The bottom one (red) lights up if the voltage drops below 10.2V. The next three are yellow and indicate voltage up to 12V in 0.6V intervals. Above them are three green ones for voltages up to 13.8V and the top one (red again) indicates a possible regulator failure leading to overcharging.
I would put one or more power transistors (maybe my old favourite the 2N3055) on the heatsink and add some extra bits to make something that behaves like a precision power zener. It's a standard circuit that you can find in any decent electronics book.If you intend going to the trouble of adding variable pitch or self feathering blades why not adopt a simpler mechanical solution: a steerable tail fin. As the turbine speed increases you drive the tail fin off to one side and thus turn the thing out of the wind.
And finally --- I would always have some kind of meter to give me a quick view of the state of the batteries. I made one from an LM3914 bar-graph driver IC and a string of eight LEDs. The bottom one (red) lights up if the voltage drops below 10.2V. The next three are yellow and indicate voltage up to 12V in 0.6V intervals. Above them are three green ones for voltages up to 13.8V and the top one (red again) indicates a possible regulator failure leading to overcharging.
