Previously-leaked images of the iPhone 6 pointed to a 1,810 mAh battery, but ESM-China news chief analyst Sung Chang Xu, citing her sources within one of Apple's supply chains, says that battery was "used solely for testing purposes."
Though a higher battery capacity usually means an increase in battery performance, this particular iPhone is also going to be bigger than previous models, so there's a chance the meatier battery will simply be used to offset the increased power needs of the device's bigger, higher-resolution display.
But as far as the iPhone 6's actual capacity, both GforGames and 9to5Mac remain highly skeptical of this particular rumor. However, a 2,100 mAh battery would not be necessarily pushing any boundaries: The Samsung Galaxy S5 features a 2,800 mAh battery that powers its 5.1-inch display, while the HTC One M8, with its 5-inch display, boasts a 2,600 mAh battery.
If this sketchy rumor is true, the iPhone 6's battery life might be similar to that of the Samsung Galaxy S3 from two years ago, with its similar 4.8-inch display and 2,100 mAh battery. That phone could reportedly handle nearly 12 hours of talk time; in comparison, the iPhone 5S can accomplish about 10 hours of talk time and 40 hours of music playing.
Hopefully we'll see some form of battery improvement in the iPhone 6 when Apple unveils the device on September 9.