I just got my hands on a RAVPower RP-PB03 5600MAH external battery the past 2-3 weeks. So far, so good. There are other, similar products for sale here on Amazon but I was impressed by the RAVPower so I did not even bother with the others -- it seems this one is small enough to carry in one's pocket or digital camerea belt case, so no problem.
Honestly, this thing is a life saver. I have the Samsung Galaxy S2 Skyrocket, and the thing eats up battery juice like you wouldn't believe. There is no way it will last even 3/4 of the day, sometimes not even half, with moderate usage. And there is so much it can do, and it's so fun, it's a shame.
RAVPower battery powerpack to the rescue! I use the REI digital camera carrying case listed below around my belt, and have the charger cord running from the battery pack a few inches to the side to my phone on its belt holster/case, and I don't have to worry at all about conserving usage because it's not going to run out of battery. Lasts all day.
That's a great thing, bc I love my new phone and I get a lot of use out of it, reading apps and webpages about my interests.
Generally, I get about three and a half charge cycles on my phone for one cycle on the battery pack. I couldn't ask for anything more.
I have already had two people I know ask me about this pack and they are getting one for themselves; likewise, I recommend it to everyone who goes through battery life quick on their cellphones.