I think the motor has much too high a cut-in RPM for 7' diameter PVC blades.
They may start turning soon, but it will take an awful lot of wind speed to get up to 12V. Like a hurricane.
PVC blades are not fast compared to carved wood blades. Reaching 400 RPM to start charging, with a 7' diameter PVC prop, is pretty crazy.
A 4' diameter blade is going to be better because it will reach charging voltage more often. Even if it takes a gust to get it going, I expect that is going to work out a lot better.
No reason to be turning in low winds or a breeze.
As long as it starts turning at 8 or 9 MPH, keeps turning at 7MPH, and is at charging RPM around 7MPH, that's about the best I would hope for.
There is no power below about 6.5MPH.
My wind is always under 6MPH or over 8MPH. Very rare to see steady 7MPH winds here, so I am happy if it gets turning in a 10MPH gust and stays charging at 8MPH.
And 8MPH is a lot more than most people guess.
Might look through Adam's treadmill ordeal. He had about every problem there could be, and he got it working in a short time. Fairly well documented, and all in one place.
http://www.fieldlines.com/story/2008/8/25/12258/0492
Try to get the blades as balanced as possible.
G-