I have a Busch Muller dynamo hub on my bike and ... when it was new it was brilliant.
Two years ago I sprang for the light that had the extra USB port ... well, the first light rain shower that failed due to poor design but I got a different brand of same thing from SIneWave cycles. I wired that on and things were peachy for a while.

Then it started to fail to hold a charge. It was acting as if, say, something was in the USB port draining from it ...
... and then a big wind blew the bike over and hte whole housing broke apart.

So... I replaced the light with the much improved newer model (which Peter White Bicycles sold me for half price given the issues with the first one, tho' it was well past warranty and I'd thoroughly violated same installing the Sinewave...)

... but ... it's doing exactly the same thing. On the moonlight ride last night, I thought I had to ride faster than I should have to get it to "bright" (it has two brightnesses), but figured it might need some time to store up some energy. Nope... 8 miles out, middle of nowhere, it's just ... going out. Nuthin'. Now, if I sped up to 15-18 mph, and on that one little hill thing, it would blaze in all its glory... then die.

I s'pose I should first check the instrucitons to make sure I haven't forgotten a step in installation, and/or see if it's sending energy to the USB port (I don't have a place to *put* the phone or I'd have done that)... or figure out if there's osmething going wrong in the actual hub??? These things are supposed to last forever...