I did that, but I used battery - couldn't figure out how to hook up to the e-bike's 50v electrical system (plus the DC-DC converter with high enough current...)

So I am using LiPo 3S, 2200mAh. Works like a charm. I keep it at its storage voltage (3.7-3.8v per cell), and it hardly drained the battery (there is no paracitic drain). Whole thing was like $20.