Will a dynamo supply enough charge to power a light AND charge a battery / device at the same time or do I always have to switch between the 2? I understand that at low speeds there won't be enough output but wonder whether there might be excess output at higher speeds?

I don't know much about electronics. However, I would have thought it's theoretically possible.

Now if it's possible with off the shelf components is another question, might need some custom bits/design.

I think on a road ride with higher speeds you could use a light and charge a battery at the same time, but you will only have 0.5 amps or so to share between the 2, so you might find the light is dim or the battery takes ages to charge.

Selecting which of the 2 you want to bias the current to might be tricky.....

There's not an easy answer to this...

I have two set ups on two bikes. Both kemo units one is the larger 172n the other the smaller 172. The n is either or depending on the switch.

But the smaller 172 outputs 300mah max to the usb. More than than and it switched to powering the lights. I've not had the inclination to check if it is still powering the usb too. However judging by how rapidly my usb battery charged on my recent trip I would say that it is powering both.

In any case over the course if a long days ride you can do enough of both to be effectively running both. Just not always at the same time all the time!

If you want your lights on to be seen then the revo (and I guess others) have enough 'stop light' capacity so on a fast road section you are able to flick to lights for 20 seconds then back to usb and the lights will remain on for several minutes.

Yep,

SP-PD8 on 700c connected to bm front and rear lights and to e-werk. Lights work fine and e-werk charges everything fine with the lights switched on.

SP-PD8x on 26". Normally only runs the e-werk, but tried it with the lights as well and it was fine at over 8-10km per hour. That light isn't good enough for off road though.