I don't think there would be a problem operating at 5V input. It wouldn't be very efficient due to your power bank wasting energy to boost to 5V needlessly.
-Garry
Needles to say original HD-016 is powered by 8.4V (2S2P 18650 cells) and moded one is designed to be powered by 4.2V (if I'm not wrong). So stock HD-016 won't run from USB power bank unless you raise the voltage to at least 6.6V.
No, my modded one runs on 8.4v. If I remember correctly, the original driver did operate on 4.2v input - pretty sure of that.
-Garry
Nope, see my review https://budgetlightforum.com/t/-/29350 . So in either case USB power bank won't be usable unless you place step-up converter in between. Pretty useless.
Thanks for the posts, like I wrote, ledoman testet at 3.4V even. Also someone said, it worked fine down til 4V. I have tried it with an 5V USB bank and that worked as well, not that brigt, but that might have been because of the low output of that bank. So it works, but does it cause harm to the buck?
I think about getting a powerbank with an adjustable boost (5-13V), so i could rise the voltage up to 8.5V, but I guess that would be more inefficient than maybe 6V?
(I know, there are losses from 3.7V -> 5-8.5V -> 3…4V for the LEDs and I’ll have to figure out the optimum for the powerbank regarding runtime, but for the sake of simplicity in handling and less devices/multiple purposes, I want to do it that way.)
Ok. I didn't remember that it can run so low.
Still, you can't run it at full power (both leds on) if your bank can't provide enough current. When you lover the voltage you would need higher current gaining some more looses in the wires. Very likely you would need somewhere up to the 3A from the power bank.
Best to try it and see how it goes. Do a runtime test with a fan blowing over it. I'm no electronics expert, but I can't see how you'd damage a buck circuit from low voltage. I understand a typical buck circuit correctly, once you drop the voltage down to required level (or lower) the circuit has no reason to "buck" any voltage and just goes direct drive (letting your lower voltage level pass straight through). It still seems a waste using the power bank, but if that's what you want and it works out so be it.
-Garry
I agree with garrybunk's comments. Should not cause any damage to the buck driver (actually 2 of them in this light when stock. One per emitter.). It will just have lots of energy losses due to 2 convertions. First, the boosting of the cell voltage to 5V and then the bucking of the voltage back down to the Vf of the emitters.
It wouldn't work in the light modded in this thread because the emitters are in series and therefore need more than 5V to clear the LVP threshold and make a usable amount of light.
No, can’t do that. Have to ride til it dies! 
Thanks for your thoughts about the electronics! I can try out now with good conscience.
Sure, there are losses due to the “senseless” up and down[1] conversion, thought about that a lot.
I bought a Qidian 3A 4x 18650 now, which has 2 USB 1A/2A and a jack out and is adjustable from 5 to 12V (about 15 bucks) and 4 Panasonic NCR18650b w/ 3400mAh (24 w/ shipping), so around 40 for the whole package and got that out of it:
- Bikelight battery pack with good capacity cells
- a charger for 18650 cells
- a USB power bank
So, the “losses” work for me, I guess, otherwise I would have had to order a 2S2P pack (around 40) and a quality charger with either a balancer (25 and up) or single cells (24), a single cell charger (10 and up) and a case for single cells (15, ok USB would be also included)). Or of course 40 for the pack and charge with the 8.4V/1A charger, not giving a … about the balancing.
Let’s see how that will work out, I can tell when the items arrive, if you want.
[1] pretty weird anyway, when you think of the 3.0V-4.2V of the cells and the 3…4V a LED requires?!?