Depends on what the generator shaft is connected to.

If you have it connected to a windmill prop
you don't need a resistor.  The windmill will
spin up (given enough wind) until the voltage
is up to where it starts charging the battery
(cutin).  Then as the wind rises more the charging
current goes up - and the current makes the genny
fight speedup of the prop.

The result is that the voltage at the battery stays
the same, the prop only speeds up a little (corresponding
to the extra voltage drop across the internal resistance
of the genny and the wiring due to the increased current),
and the extra force from the wind fighting the prop and
genny turns into extra current charging the battery.

You design a furling system (such as an angled pivot
or hinge on the tail mount) to make the mill turn
away from the wind when the charging current is
approaching the max the genny can handle without
taking damage from the heat generated by excessive
currents.  (It takes a little time to heat it up,
so the genny won't fry before the mill turns away
from a gust.)

You also add some "dump load" when the
battery gets full, to drain away the extra energy that
would otherwise overcharge and damage the battery.
(You don't want to disconnect the genny from the battery:
That will take the load off the prop and let it overspeed,
tearing it apart even moderately high winds.  When you
have to unhook the genny from a load you short its output
to make it act as an electric brake and keep the prop
nearly stopped.)