Many people ask how you can connect an alternator or generator that puts out 30v (into an open circuit) to a battery bank that's only 12v without frying the batteries. The batteries actually regulate themselves and pull the generator voltage down to their charging voltage, UNTIL they get full. Then the power needs to be diverted somewhere else. That is the job of a diversion charge controller and the dump load it sends power to.
Dump loads can be as simple as turning on more lights in your house when it's windy and your battery bank is full. Electric heatings elements are often used. Many people buy or build an automatic diversion charge controller that senses your battery bank voltage and turns heating elements or other loads on to dump power.
Homemade units:
http://www.fieldlines.com/story/2007/8/24/172521/889
http://www.bioelectrifier.com/charge.htm
http://www.homepower.com/files/shuntregulator18.pdf
Some commercial units can operate as dump load controllers, for example:
http://www.xantrex.com/products/product.asp?did=191
(note -- they can be set to function as a solar charge controller or as a wind dump load controller, but not both at the same time.)
Some good discussion on diversion charge controllers and dump loads:
http://www.fieldlines.com/story/2003/7/18/8715/42490
