I was sent an Elecjet charging power bank to try out. It's a 10,000mAh Graphene battery with a digital meter that indicates the remaining charge as a percentage. When I plugged it in for the first time to charge it (using the supplied 100W Charger) it had about a 50% charge. In less than 20 minutes it was at 100%.
To test it out, I used to power my MacBook Pro through the USB C port, and it quickly topped off the Mac's internal battery and kept it running for several hours longer than it would without the charging bank plugged in it.
Since I've only had it for a few weeks, I can't say anything about its duty cycle, but with limited time with it, the charging time and capacity are impressive. It's available for pre-order on Indiegogo.