My iPhone battery indicates that it is fully charged. Then it suddenly dies.

[iMore Question]

Is the problem with the battery or is it the indicator?

 

[phreddyl]

Well that depends- how long does it last after being on the charger for a few hours? My guess is your battery is shot

 

[happyed19]

After owning my iPhone 4 and my iPhone 5 for a year and a half they started to shutdown when the battery level was still at 10-20%. They would also turn back on and be immediately at 50%. This is a very good indication that your battery needs to be replaced. If this is occurring for you on your iPhone 6, the battery replacement process isn't too difficult. I found this video the most helpful. https://www.youtube.com/watch?v=demKhXKBg2Q