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^that one is way to bulky , and requires adapters, although it recharges more, it look uncomfortable to carry around
 
There have been all kinds o fdiscussions about backup battery solutions in the accessories forum.

I just bought a backup battery extender off of Ebay for $20 (the kind that folds down and fits the iphone kind of like a case). I tried it out yesterday and seemed to work great. I went with that over the one in the OP because it holds more juice and should be good for more than one charge.
 
I've got a RichardSolo battery (not the latest version) and it can basically take an iphone at 10% warning to almost full

New one on their site claims it can take an iphone from 0 to full and then a bit more (1800mah battery)
 
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