Astro 2 external battery by Anker is one of the most powerful, versatile and reliable external batteries that I’ve come across. It has met or exceeded most of my expectations for such a device. I would strongly recommend it as a travel backup battery for most small electronic devices.

***** The Specs *****

This 8400 mAh battery has one DC 5V 1A input, and two outputs: DC 5V 0.6 A and DC 5V 1 A. The specified input charging time is 6-7 hours, and in my own test I’ve verified this to be an accurate estimate. The total number of rechargings is specified to be 500, but for obvious reasons I’ve not been able to test this claim.

***** Battery Capacity *****

At 8400 mAh, Astro2 is the largest-capacity external USB battery that I’ve ever tested. It has enough capacity to charge an iPhone or any similar small electronic device at least a couple of times. For a less power-hungry device (plain vanilla cell phone, Kindle, etc.) you can expect to get at least full charges from this battery. The two charging outputs have different charging rates, but either one will be able to charge your device in a two to three hours max. I have tested the charger with Morolar Razr, Motoroal Krzr, Amazon’s third and fourth generation Kindle, iPhone 3G, iPhone 4S, and the first generation iPad. It performed more than adequately with all of those devices.

In order to get a good, intuitive feel for the charge capacity of this external battery I subjected it to the “iPad test.” That is, I try charging my first generation iPad form the fully discharged state in order to see how much “juice” am I able to get. iPad is the largest electronic device that I can think of that would still qualify (barely) into the small-electronics category. (A MacBook Air would definitely be out.) The verdict: I was able to charge the iPad to about 56% capacity. This is, thus far, the most I’ve been able to get out of any such external battery. It took this battery about two hours to charge it to this level. This means that you’ll be able to watch several movies on a trans-oceanic flight and still be able to use your iPad when you land.

***** Accessories *****

The battery comes with a built-in power adapter, two USB cords, one PSP tip, one Nokia tip, and a micro and mini USB tips. This versatility of adapter heads means that you’ll be able to use this battery with most cellphones and small electronic devices that are popular right now, but the owners of the iOS devices need to remember to use their own Apple USB cord. I wish that the battery came with a small carrying bag or a pouch to put all these accessories in, as I find myself losing these kinds of accessories more often than I care to admit. The battery has a small built-in led flashlight, but I personally don’t have much of a need for such a thing.

***** Design and Build *****

The battery sports a slick cool design, and it looks and feels very modern. It doesn’t feel cheep, and it will definitely not look out of place with the latest and coolest electronic gizmos. It’s a bit on a heavy side, so I’d not recommend carrying it in your pocket or in a purse all the time.

***** The Bottom Line *****

If you are a frequent traveler and carry more than one small electronic device with you all the time, or tend to go on longer trips to places without an easy access to a wall outlet (camping, long flights) then this charger is a must for you. It will keep all of your devices charged for a reasonably long periods of time, and it will give you a peace of mind that you’ll never be in a situation where you run out of power. Astro 2 battery is compact and durable, and it will serve well all of your portable charging needs.