The thing will building turbines is you can buy a motor on ebay that is listed as 'for wind turbine' only to find out that its a DC motor, but needs to be spun at 3000 rpm to make 12 volts. Most horizontal axis wind turbines work in the 300-900rpm range, and the bigger the blades, the slower they spin, but the more power they could capture from swept area. Treadmill motors work well for 12 volt systems, as they normally run at 240 volts dc at 3000 rpm. Scale that down - so, 3000rpm divided by 240 volts and you have 12.5rpm per volt. Realistically there are parasitic losses, heat, friction, etc, so to charge 12 volts, you need to spin maybe 20% faster than you calculate. 12.5 rpm multiplied by 12 volts plus 20% is 180rpm. Which isn't too shabby for a cut-in (charging) speed. A set of 3 gutter blades, approx 2 feet long would work well.
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