I have a Mi Power Bank 10400mAH. I got it three months ago, about $15.
First, I like the large capacity.
When I use it to charge my cellphone, it can do about 4 full charges. Per specs from official site. It said 4.5 full charges on the iPhone 5s. 1.5 full charges on the iPad mini.
On Specs, up to 93%, On the body of bank, it showns > 90%. So I can say the conversion rate is between 90% to 93%.
The four holes between the power button and micro usb port are indicator lights. When four lights on, it means full amount. When three lights on, 75%. Two lights on, 50%. One lights on, 25%.
To charge this Power Bank, it tooks 5.5 hours on 5V/2A adapter. Or, 10 hours on 5V/1A adapter.
Official site http://www.mi.com/en/pb10400/