I've discovered that the iPhone 5 battery usage estimation is not properly calibrated within iOS 6 from the factory. Some have been complaining online that their battery doesn't last long, when in actuality they have much more battery to go. Basically, the phone will sit on 1% and not die for 15-30 minutes after the fact, depending on your usage with it at that time. Normally, when an iPhone gets to 2 or 1%, it'll auto shut down. This tells me the % battery remaining is more likely around 15 to 25%.
Most likely, once you allow the 5 to fully drain and shut off, and then recharge it fully, it'll start estimating battery usage properly. I just finally was able to drain my battery fully, after it being on 1% for the last near 30 min. I'm now fully recharging it, and I'll report back how the battery usage estimation is afterwards. In the meantime, I'd suggest everyone with the 5 to allow it to drain completely and shut off, THEN do a full recharge before unplugging it. This should correct the issue, and will allow you to see that the 5 does indeed have the great battery performance that Apple promised.
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I hope this helps!