Not only does the 3.7 V, 1750 mAh battery power the phone, but the user manual states that it also doubles as the NFC antenna.

O RLY? We had to see this one for ourselves.

Lo and behold, there's a sweet antenna hiding underneath the battery's shiny wrapper. So if you ever want to buy a replacement battery (and keep NFC functional), ensure that the battery has the antenna.

Our battery says "Near Field Communication" on it, so that might be a good indicator if it will function as an NFC antenna.