Overly large CSS pages and other unused website code may drain your smartphone's battery.
http://www.notebookcheck.net/Bloated-Code-May-Drain-Your-Smartphone-Battery.74149.0.html
I recently bought one of these from Kinkoo: http://www.ikinkoo.com/shop/portable-charger/kinkoo-infinite-one-8000mah.html
I have found it to be great. With real 8,000mAh can charge iPhone 5 for 4 times, with juice still to spare. It's very light to carry every day(only 5.7 oz).
They ship by Fedex(free shipping), very fast to get it. Highly recommended.