New Trent Easypak 7000mAh Portable Triple USB Port External Battery Charger/Power Pack for Smartphones, Tablets and more (w/built-in USB cable)
I bought this to use with my iphone.  I have one of the golf GPS apps installed on the iphone, and these apps really suck the battery.  This unit was the perfect solution.  I typically hook it up to the iphone on the 10th hole, when the iphone battery is starting to run dry.  I have used the external battery for the back nine twice, and it still shows a full charge.  Pair this battery with the Bracketron cupholder mount and you are good to go golfing.