When it comes to external batteries some of them come quite cheap but they come nasty to in a affect that they don't not charge your device very fast a lot of these charger say they can charge a iPad but it takes 5 to 7 hours roughly to charge a iPad which isn't really much good if you need power in emergency and waiting 8 hours for it just doesn't fit the bill at all really.The problem is this external battery only 1.2 amps which really is just enough power to charge your iPhone and it would charge it about 3 hrs roughly but when it comes to iPad it would take about 6 hrs to charge it or maybe more.
The correct amount of amps you need to charge a iPad is 2.1Mah and this correct amount will charge you iPad in about four hours roughly.
I hope that my knowledge is some use to you because i had the problem in the begin and i had to find out for myself and find out why the iPad charged so slow and it stated that it can charge a iPad which it does but not very fast at all quite slow in fact not good if you need power.
I have a good charger but it is expensive but it charges a iPad really well roughly it charges my iPad mini 30% a hour roughly
The best advice i can give you is be really carefully which charger you decide to invest in because you could have the problem where i had it take ages to charge my iPad
Here is the site what supply the battery that i use they are very good quality and engineered very well i'am actually going to invest in another because i have two iPad to that i can keep them both charged up all the time and charge my iPhone when i am out and about.
A lot of photographer use these batteries to keep that laptop charged up when they are out in the field and need power i have used the mine to charge my macbook up a few times
USA Site
HyperJuice External Battery for Apple MacBook, iPad, iPhone, USB Devices
This is the version that i have to keep my iPad charged up when i'am on the move
United kingdom site
External MacBook Batteries and Portable MacBook Chargers from Protronica Ltd
I like i have said just be careful what you buy because its not much a iPad charging very slow
If you have anymore questions and you need some help please feel free to contact me on the forum
and i will be willing to help you out of my best of my abilities.
Best regards paul
Happy ipadding