iPhone 6 Battery Capacity

All rechargeable batteries have a limited lifespan and may eventually need to be serviced or recycled. Your own battery’s lifespan will vary depending on how you use your device as well as the settings you choose for it.

Your battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles. The one-year warranty includes service coverage for a defective battery. If it is out of warranty, Apple offers a battery service for $79, plus $6.95 shipping, subject to local tax.

http://www.apple.com/batteries/service-and-recycling/

Your maximum capacity has naturally depleted within the expected means, and has not depleted below 80% of its original maximum capacity. You can ask Apple to run a diagnostic on the battery to see if the battery has failed, but judging on the information you already possess, it doesn't seem that that is the case. In your position, I would hold off on replacing the battery for presumable 110 mAh loss and instead maybe adjust some settings to increase the performance of your current battery. http://www.apple.com/batteries/maximizing-performance/

How are you measuring that? Apple does not expose Battery Management API in their SDKs, so if you are using a third party app, it's likely inaccurate. If you experience that your battery life is shorted than before, follow first of all these tips:

http://www.apple.com/batteries/maximizing-performance/

Should they don't work, and your battery could be damaged, get an appointment at your closer Apple Store so them can review and diagnose your device

https://getsupport.apple.com

I have a Mophie case with a charger built in. I love it!! Might want to give that a try!