I had lots of mixed success with various powerbanks until I settled on one that dealt with pass through charging as well as not needing a constant steady source.
Settled on a 10000 mah zendure. Figured I wasn't gonna charge it any more than that in a day.
Also phone doesnt charge so well attached directly to dynamo as stopping/starting doesnt do so good. Better off using the power bank as an inbetween if charging whilst attached to the dynamo. Lotsa newer power banks dont do pass through so make sure you get one that does if you go down this route.