[Renate]

Maybe I've stated that poorly. Every device requires large start-up current that is usually in excess of what the USB can deliver (unless it's a Type C wih PD). In any case, what I meant is you can't just connect a power supply to red and black of where a battery used to go and get the thing to boot (unless it only uses two wires). The 3rd and 4th (if present) have to be connected to something that say, "I'm a battery!" Here's a photo of my Verizon hot spot which I'm using right now. Before it ran hot (which increases noise in the front end, killing performance in marginal areas). I removed the battery, the front window, the display and installed a heatsink. It runs much cooler now.

Here's a plot I did of something (I forget what) but you can see that it peaked at a good 1.4 Amperes when booting! Most devices hit at least 1 Ampere. Edit: I think that's my Nexus 7. I checked my Moto E5 and it hit an Ampere.