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a friend of mine tried a 3rd party battery charger, and it fried his phone.i would advise against using anything that doesnt guarantee the safety of ur phone.
 
I've been using the Richard Solo backup battery for a few years now on a 3G, 3Gs, and now my iPhone 4 and it works VERY well. I just charge it and forget about it until i need it.

I've used the mophie juice pack on the 3G and 3Gs and didn't like it because it really does block your iphones phone signal. That's the only one that I would recommend against.
 
I use a Satechi. It's a bit more than the one you listed but it works great and seems to have a pretty good rep as far as phone compatibility.
 
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