'Power Banks'/emergency chargers are very common these days, of course, with everyone benefitting from a portable source of USB power when faced with a smartphone or other gaddget with low battery. But performance, construction and value do vary wildly, which is why I attempt a degree of curation here on AAWP.
In this case, the Xiaomi Mi 16000 Power Bank is superbly constructed, with premium aluminium shell - cold, hard metal in the hand that feels like it means business - and it does. In the box are just the charger and a short USB to microUSB cable, to get you going. Plus an instruction manual - in Chinese! Still, it's not rocket science to operate this sort of accessory...
On one end are the ports you need: a big power button, four status LEDs, of which more later, microUSB 'in' and two 2.1A USb output ports:
The fit and finish is terrific, as you'd perhaps expect from Xiaomi, one of the biggest names in China in terms of smartphone electronics. The ability to output from both ports at 2.1A means that, in theory, you can charge two tablets at the same time, which is impressive. I tested this with a couple of iPads, with success.
Most of my testing was with smartphones though - I managed to fully charge two Lumias, one 808 PureView, a Nexus 6 (twice) and still had charge over to top up an iPad mini, all from one charge of the Xiaomi power bank.
It's at this point that I should clarify the naming scheme used by these power banks. The clue here is actually explicit, in the labelling on the other end of the Xiaomi accessory:
Now - look closely. The '16000mAh' legend in huge type is at 3.6V, i.e. the internal, native voltage of the cells inside the bank. In order to charge something else (i.e. a phone), this is ramped up electronically to 5.1V, at which the capacity is effectively 10200mAh, i.e just over a quarter less, minus a little more for inefficiencies in the conversion process. This subtle naming issue is common to just about every other power bank out there (the larger number always sounds best on the page!), so well done to Xiaomi for making the distinction clear, even if the fonts used here are slightly misleading!
In my testing, I really did get 10200mAh of output - in fact, I measured it at 10500mAh, though with +/-500mAh, but impressive considering that many other chargers fail to live up their claimed capacity.
But one caveat over the status LEDs. As usual, they're there to show 0-25%, 25%-50%, and so on. But when the button is pressed (e.g. to check charge), the LEDs first all flash on (presumably to check that they still work) and then the actual charge is shown. So all four and then (say) two. This is fair enough, though do note that when the power bank is totally drained it will still show the four LEDs followed by nothing at all. So the casual user might be fooled into thinking that the power bank was full when in fact the opposite was true. Worth noting anyway. I just don't think there's a need for the 'status check' of lighting all four each time.
And the gaffa tape? It's needed to cover the four LEDs if you use the Xiaomi Power Bank at night. You see, all the while charging (of the bank or the bank to other devices) is taking place, the LEDs flash. In a dark room, e.g. while sleeping, this flashing is surprisingly bright and could be annoying. Thus, while travelling and using the accessory overnight, a small strip of black gaffa tape is a good tip, just keep it on the underside of the charger and you're good to go!
Highly recommended overall though. The aluminium 'Xiaomi 16000mAh Power Bank' was supplied for review by the kind folks at GearBest, which ships all over the world. Note that it's also available from Amazon UK, e.g. here. At $30 (or equivalent) it's not the absolute cheapest solution out there, but it's still super value and you're making completely sure of quality.