Have one of these, as aging nexus 7 battery life is on the way down, very impressed, charges it nearly 3 times over.
Also have the smaller version so I don't have to get phone replaceable batteries anymore for old galaxyS2, also been very handy.
(www.amazon.co.uk/gp/product/B00P8SY7HQ?qid=1448344189 - currently only £6.59 as well)
[Upgraded Capacity] Anker Astro E7 Ultra-High Capacity 26800mAh 3-Port 4A Compact Portable Charger External Battery Power Bank with PowerIQ Technology for iPhone, iPad, Samsung and More (Black)
The Anker Advantage: Join the 10 million+ powered by Amazon's #1 charging brand.
Exclusive PowerIQ Technology: Detects your device to deliver its fastest possible charge speed up to 3 amps per port or 4 amps through three ports.
Giant Capacity: Charges the iPhone 6 ten times, the iPhone 6 Plus or Galaxy S6 over six times or the iPad Air twice. Safely recharges with a 2 amp or higher output charger (please note most phone chargers only have 1 amp).
Superior Quality: Premium panasonic cells, cutting-edge circuitry and a sturdy gloss exterior ensure reliability and safety. To activate the LED flashlight, hold or double click the power meter button.
What You Get: Anker Astro E7 Portable Charger External Battery Power Bank, Micro USB cable, travel pouch, welcome guide, our fan-favorite 18-month warranty and friendly customer service.