Had a chance to test the battery today and wanted to share:
tldr: I set it up to draw ~10watts and to my surprise, it actually lasted 5 hours! Actual total power consumed: 46.9Wh
During testing, the voltage fluxuated between 4.257V and 4.396V. Not fantastic, especially when you consider that the official USB spec is 4.4V to 5.25V so this was never even in the range. The voltage regulator in this pack is definitely not high quality and you can see that in the very stepped graph in the Google Sheet linked below. It also gets worse as the battery drains towards the end. Note: the digital readout on the battery is pretty useless (last "15%" should have lasted 45min but was instead drained in apx 5min)
Unfortunately I had some trouble with my constant current load so I had to settle for some power resistors. Over the 5 hours of testing, the power draw was between 9.12W and 9.70W averaging at 9.36W so it was pretty consistent. One pleasant surprise: the battery never got hot or even slightly warm to the touch. Same with during charging.
I went back to look at the product page and noticed that while it says "20000mAh", if you look at the details, it also says 64Wh which means those 20kmAh were being measured at the cell level which is only 3.2V. This equates to only 12,800mAh at the 5V the battery should output. Not sure if that counts as false advertising, but it's certainly misleading... then on top of that, with a ~10watt load, I wasn't even able to eek out 47Wh.
So, how good or bad is this thing? Well, if you were expecting a 20,000mAh 5V battery, you'd be pretty disappointed. Actual capacity is under half of your expected 100Wh and I hope whatever devices you connect have good power regulation. But if what you expected was 4 LiFePO4 cells with a usable 3672mAh each for $20 total then you're in luck my friend.
I personally am thinking about buying another one of these and a 4s BMS to make my own 4s2p pack. Only issue is I can't figure out how to take the darned thing apart. I may have to just completely destroy it to get in.
I used a logging multimeter to measure power draw (on 5 sec interval) so if anyone's interested, all the data can be seen here:
Tqka 20000 Battery Testing