what happens when you charge your battery is that you are using electrical energy to effect a chemical change. If the 'internal resistance' of your generator is so low that it can raise the battery above let's say 16 volts or so then you are probably using way too heavy a generator for your charging.
But, the internal resistance of your battery is probably much lower than your generator so it will win the 'tug of war' as soon as you connect it. Which means that with the battery connected the voltage will drop a lot, and current will start flowing into the battery.
If this current is too high, or if it continues at too high a level once the battery is full you will damage your terminals/plates or 'overcharge' your battery to the point where the water in the electrolyte boils off.
To avoid that from happening you can simply shut down your windmill when the battery is charged, or you can automate the whole process by using a dumpload.
JimU keeps a nice schematic of a charge controller here
What it does is divert the power coming from your windmill to a 'dumpload' as soon as your batteries are full.
Dumploads are usually resistive heaters, they can sink a whole pile of power in a fairly compact package.
hope this helps. good luck !
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