Power banks come in all shapes and sizes, but the Nitecore 21700 intelligent battery system is one of the smallest, most bare-bones I've used.
It's basically a super-powerful rechargeable battery, a charger/power bank adapter, and a lantern attachment.
Let's take a look at the three main components (the case is, at the end of the day, just a nice plastic case), starting with the battery.
The cell at the heart of this is a Nitecore NL2150HPi 5000mAh/3.6V/18Wh 21700 rechargeable cell. This battery can put out 15A continuous discharge and features a number of built-in protections.
The cell is also interesting in that it has positive and negative terminals at both ends.
It's a beast of a cell.
Then there's the MPB21 magnetic power bank adapter. This clicks onto the top of the battery and features a USB-A port for output and a microUSB for charging, both compatible with 5V/2A.
There's also an ML21 magnetic lantern attachment. This offers 80 lumens of >90 high CRI soft light and can run for 19 hours.
And that's it. The package retails for under $40 and is a great minimalist power package.