Yell Energy SQUARE 4000mAh Power Bank
Yell makes Portable Batteries, which can charge your Phone and Tablets; they do various chargers from small battery packs to bigger ones, which can hold more charge for more long lasting charging.
You can get Yell chargers in a choice of colours,
I found in my testing that Energy-Bar-4000mAh took 3 hours to charge from the mains, and charged Iphone 5 at battery life from 10% to 97%.
This is very good to have in your backpack, Pocket, for them times where you need to make emergency calls, or get that important email sent out
Device is solid and well built, and very affordable