My Winter, black Ice bike I use a 6v 10 w battery light , Niterider Sport,
as i go slower the battery will maintain light.

Just raining I have a SON hub dynamo and a e6 schmidt and wired taillight,
there is an e6 secondary.. it goes in series . no off, just on or thru.

good for doubling the light for fast downhills, but you have to be moving at a good clip
to power it.
good for not overrunning the lights on descents , but on the flats the rider strength
to break and maintain 13mph is needed.

So I might shed the E6 secondary I got. I'm just not that fast ..