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Thanks for all the comments. The windmill has kept the stock tank full for 30-40 years, so the wind must be pretty strong. I was just thinking I could take advantage of an existing windmill (on the cheap).
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Mike, I was watching a modern windmill today, with lots of blades and turning relatively slowly. There wasn't much wind.
It was located at some large ponds and I couldn't tell what is was doing besides pumping water.
When you think about RV pumps and the relatively low current at 12 vdc and the impressive amount water they can transfer, with relatively low head, this suggests that the type of windmill we're talking about -low RPM, high torque, small horse power, would likely not be able to generate a lot of watts for battery charging.
There are specialty alternators, 3 phase usually like auto alternators, but using permanent magnets for the field, [see Google or ebay] that might make useful output if you can use a gear or belt transmission to step up your windmill's RPM to run them.
This is just a guess, I'd say the windmill I looked at today, good sized and nice condition, would have ~1/3 of a HP available [after losses] to drive a specialty PM alternator.
If you reviewed the link I posted above, you'd see I get about 100 amps at 14 vdc from a truck alternator driven with abt 3 HP.
If you look at the lift [feet] the water is raised, and the volume of water pumped, you can easily calculate the work being done by your windmill.
I think it is feasible to generate a SMALL amount of power with your windmill, and considering it doesn't cost anything to do it, and if the wind is reasonably constant, you will probably have a useful means for a small amt of juice.
The trick will be the gearing and using the correct permanent magnet alternator.