The Anker 2nd Gen Astro 3 is No Longer Available as of 2016This is currently our favorite robust USB external battery charger. It will charge a smartphone 4-6 times and a tabled 2-4 times. More importantly, it charges fast. Most external batteries, including the first generation Astro, charge at a maximum of 1 amp while the 2nd Gen Astro charges your device at up to 4 amps. In additon, the input power is 1.8 amps while many other external batteries have an input power of less than 1 amp. What does this mean in the real world? It takes about 2 hours to charge an iPhone 5 compared to 3-4 hours with most external batteries and 1 hour 40 minutes from a wall charger.
You can charge the Astro from anything you can plug the included USB cable into: wall socket, computer, your car, or a portable solar charger. Best of all, you can charge three devices at once.
If you want to save money and weight, check out the Anker Asto Mini 3000 mAh. It does not charge as fast as the 2nd Gen Astro and only charges a smartphone 1-2 times, but it comes with a flashlight and is about a 1/3 the weight and cost.
Another option if you are away from a power source for a long time is the Bushnell Bear Grylls SolarWrap Mini. It has a small internal battery but charges but can be charged from the sun when needed and is relatively light.