Let me start by saying I'm very impressed with the battery life on this 4600mAh extended battery from Mugen Power!
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I had two spare (stock) batteries with my Galaxy S Fascinate and it sure was convenient having the extra power when I was out fishing, camping or simply didn't have a power supply handy to recharge my phone. Being that the stock battery on the S3 comes with a 2100mAh battery, that's usually plenty for the average smartphone user to get through their day without having to worry about recharging until they got home from work, school etc. Being a self proclaimed power-user, my Fascinate usually needed charging by 1 or 2pm. Now that I have 4G on my Galaxy S3, I find myself using my phone even more. From listening to my daily 4 hour podcast during the week, YouTube videos, texting, surfing the net and participating on forums throughout the day, I was still able to get a good 10 hours out of the stock battery. However, by the time I got home from work (sometimes sooner) my battery definitely needed to be recharged. But, with this 4600 extended battery, it easily lasts from the time I wake up (6am), until the time I went to bed (10-11pm). I easily get 23+ hours of battery life with this battery and still have about 30-40% battery life remaining. For your average user with medium usage, I'm sure you could easily get two days of life with this battery.
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Another good thing about this battery is that it comes with NFC (near field communication) built in. I can't tell you how many reviews and comments I've read on various forums where people have purchased a $15-20 dollar extended battery off Ebay, only to have it give them less battery life than stock, or maybe an hour or two more. Nor was NFC included, which is very important to have in my opinion as I use the S-Beam feature on my S3 all the time.

When it comes to smartphone batteries, you definitely get what you pay for. It comes with a replacement battery door/back cover that has a built in kickstand. (Very convenient if you like to watch movies on your S3.)

Now the "cons": Honestly, this battery is huge. It's twice the size of the stock battery. Even though it adds some width and weight, it is actually quite comfortable in hand.
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It makes it impossible to find a case that will fit the phone when using this battery, but Mugen does offer a nice leather protective pouch for it.
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Recharging this thing does take awhile, but since I usually charge mine overnight, it's not really an issue.

Aside from that, this battery is amazing! If you need an extended battery that will easily last a full 24 hours, this is the battery for you. They also offer a 2300mAh battery that doesn't require a replacement battery cover and offers at least 10% more battery life over stock.

The price of this battery is $87.95, but includes free shipping. Mugen Power is one of the top companies in the world that offers extended batteries for portable electronics, and they have a battery for pretty much every device out there. Their customer support is also top notch!
If you have any questions, please don't hesitate to ask.