Still a very broard question, however the point at which a battery begins to charge is once the charging source exceeds the battery volts. So lets say your 12v battery is at 12.5volt, then any voltage greater than that will cause your battery to begin charging.
If you are using a wind generator you will need a blocking diode that will stop the battery back feeding the turbine so that's an about 0.6v drop. So for the example I gave your wind turbine would have to produce 12.6 +0.6v =13.2 to begin charging the battery.
Now wind speed, that's another how long is a piece of string, but most turbine start producing at 3m/s, you could build a generator to do what ever you want, but if you use a ready made generator you have an abatory figure set for you.
Google this board for "power of wind" to obtain how much energy/power is in the wind at a given speed.
You realy need to explain your project to receive specific advice.
Where in OZ are you, i'm in S/W WA.
allan down under
