Hi joff,
I agree with McGuinn,
they're ok for occasional use but anything on a regular
basis and the tyre suffers.
Have you considered a hub dynamo?
They dont damage anything and the 'drag' is a lot less
than the bottle type.
In answer to your question, yes you could charge small
batteries using the bicycles dynamo.
The small bottle type dynamoes are strictly speaking
not dynamos at all, they are alternators.
They wont deliver much current, i think the windings
may be quite thin wire.
You would need to rectify, probably a diode bridge,
and also put some sort of voltage limiter across the
battery, maybe a zener diode. dont worry about a series
resistor, its own internal resistance limits its max
current to about half an amp (or less).
Regards, John