On Sun, 27 Nov 2011 14:56:00 -0000, "Ian Field"
wrote:

Tinkering with the old Sturmey archer bicycle dynamo, I was wondering
whether it would charge a 1.2Ah SLA any better if the loading was modified.

The generator is rated 6V/3W, but off load at a decent rate of knots it can
produce over a couple of hundred volts.

What I was wondering was whether its possible to get more energy into the
battery by letting the generator output voltage stretch its legs so to speak
and convert the excessive voltage down with a buck converter.

Can anyone advise on the practicality of this please?

Thanks.


You'd have to experiment with different load resistances to see what
it can deliver.

Alternators like this are often designed with a lot of internal series
inductance. As speed increases, open-circuit terminal voltage
increases, but the series inductance impedance increases too, tending
to make them constant-current sources, ideal for driving light bulbs
at sorta constant brightness at various speeds.

So I'd think that you can get more than 3 watts at higher spin speeds,
with a proper load. A series capacitor, to resonate out the inductance
at some speed, would be interesting.

Given an AC source with internal series inductance, a conjugate load
(resistor in series with a capacitor) maximizes power transfer.

Experiment! Measure!

John