There is a magic point where everything comes together.
The voltage, amps, RPM, blades, wind speed, etc.
Connecting a decent 24V windmill to a 12V battery will make it reach cut in too soon.
Then amps start to flow, which puts load on the blades before there is enough energy in the wind to remove any energy from the wind.
The generator wants to extract 20W where there is not 20W available.
This causes stall.
A simple demonstration of stall is with almost a 0V load. Shorted output.
A decent windmill will violently slow down, like it has brakes. It will rotate at a very low RPM. This is the extreme example of stall.
I expect the motor in your windmill is fairly high resistance, which makes stall less obvious, and makes the windmill less efficient.
The blades are overly powerful for the output, so the blades sort of use brute force to keep pushing when stalled.
Typically, the magic point should be when it is charging at 1ma in 6.5MPH wind. Whatever voltage that happens to be is the voltage the system should work best.
Your blades seem pretty fast. The 6.5MPH RPM is about 600RPM, so cut in voltage should be reached at about 600RPM.
The generator makes about 5.1V at 600RPM.
Therefor, working with what we have in hand, the optimum battery voltage should be 5.1V.
All that assumes the blades are a good match for the motor (they most probably are not)
and the motor is relatively efficient as a generator (it most probably is not).
That is greatly over simplified.
G-
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