Battery life percentage problem

I have had similar issues and was able to restore accuracy to my battery percentage by performing the steps in this article. It explains what might be happening, why it might be happening, and a fix for the problem.

Using the battery level meter in this manner is comparable to using your car's fuel gauge to calculate miles per gallon. The only thing that matters is the total amount of operating time from full charge to auto-shutdown.

Use the wall-mount charger that came with the iPhone and charge overnight. Do NOT use a computer's USB port. Then, operate it normally until auto shut-down (ignore any low level alerts that may appear). An irony is that doing that test to determine the total operating time is also the procedure necessary to calibrate the battery level meter.

I'm not claiming that you do not have a problem. I am stating, however, that we don't yet know. If the above test does, in fact, indicate a problem, read this.

Also, according to Apple:

Use Your Device Regularly

For proper reporting of the battery’s state of charge, be sure to go through at least one charge cycle per month (charging the battery to 100% and then completely running it down).

Elsewhere, Apple elaborates and explains that two half-discharges (or four quarter-discharges, etc.) equals one full discharge.