Dynamo Lights

If I leave my AliExpress special (light) on it seems to flash rather than build up capacity, until I switch it off and on. Indicative of something about to go wrong or is this normal?

I think I fixed this - looks like it might have been caused by a short on the rear light cable when it was damp (which I shortened as I'm not using). I isolated the cables from one another properly and it doesn't seem to have any issues now.