In most hydroelectric and wind-powered battery charging systems, the charging source cannot be disconnected from the battery while running without the possibility of damaging them from over-voltage.
The typical way to regulate battery charging voltage with this type of generating system is to use a "load diversion" type of charge control. The Morningstar TS45 & TS60 and the Xantrex C-35,40, and 60 can be configured for this mode of charge control. A diversion-type charge control also may be used in a PV system. If the array is much larger than the nessasary to charge the battery, excess power can be used to heat water or air.
To determine wattage use Ohm's Law, V= iR or Voltage=Amps*Ohms