I have been using an external battery with 2 USB outputs for a year or so. Mainly for charging my phone when on an extended mission away from USB power, e.g. walking around or on planes without seat-power.
I just upgraded from a smaller using to a really nice piece of work – an Anker Astro 12Ah battery with 3 outputs. The capacity numbers are a bit silly because they don’t say how much energy (e.g. Watt Hours) and you can’s compute that without the voltage they use for the batteries. Nevertheless this unit will charge a Galaxy Note Tablet twice or a Galaxy S4 or Note 4 times at least – so nice and juicy.
The power INPUT (Charge) is microUSB which means compatibility with the “world standard”. Use the same cable for charging and powering.
The main reason I went for it was the amazing price on Amazon:
I have been using it for 1 month now and it works really well – powering a Note 3 and a Note 10.1 (2014), Bluetooth headset, Parrot Zik headphones and various iPhones, iPads etc that friends or colleagues need boosting.