A computer power supply has a constant voltage output, which is not good for charging batteries, nor does it give a particularly good match between the generator and the load.
You can use a switch mode power supply (smps), but you need to vary the pulse width modulation (pwm) such that the battery charging current varies with the power available from the mill.
Ideally, you need a maximum power point tracker (mppt) algorithm, which constantly varies the pwm from moment to moment to match the power available from the mill.(1)
With the Fisher & Paykel (F&P), the maximum power available is a linear function with rpm(2), due to iron losses and internal inductance mainly; therefore you can use an LM2917 tacho(3), which gives a linear output voltage versus rpm, to control the pwm. By adjusting the slope of the transfer function, you can get it to match(4).
(1) I have a working but not fully tested prototype, see my diary.
http://www.fieldlines.com/story/2006/3/12/14840/1315
(2) There is a pdf of a spreadsheet I did after extensive testing of an F&P.
http://www.otherpower.com/images/scimages/2006/09590030sxc.pdf
(3) Again, in my diary
http://www.fieldlines.com/story/2004/8/22/212629/116
(4) Disclaimer; I haven't physically done this, but I am helping someone who is about to do exactly this.
Amanda
