I don't see why not, but cannot confirm. https://www.google.com/amp/s/amp.reddit.com/r/NintendoSwitch/comments/84unmj/using_a_powerbank_with_your_switch/
I hope people don't decide to open up their damaged powerbanks based on your comment and end up burning the house down. Silly thing to post IMO.
I think a 20000mah pack would take 10 hours to charge with a standard 5V 2A USB charger. But I don't totally understand electricity.
I have one of these. It fell awkwardly when it was charging and completely wrecked the micro USB port. I opened it up to see how easy it would be to replace the port. It was so much aggro to open and now it is I can't get the circuit board out as it appears to be stuck down. I'd put a bit of force in but the cells themselves are spot welded to the circuit board so I've no clue how to disconnect them either. So yea, try not to damage it because unlike Anker they won't just replace it.
@surfer69 "Had one from Banggood a year or so ago. It stopped charging" The same thing happened to mine... first of my Xiaomi purchases to let me down. I make it down to the Xiaomi shop...