my iphone 6 plus 128gb battery behaving a lot strange, sometimes my phone gets off at 26% and when i plug in to the charger it starts and start to charge from 26%, and sometimes it happen with 15% with same charging thing, but what i have seen that from 15% to 50% it charge within 30 minutes only and after that the charging goes slow, i visited service center and they diagnose it and found battery and everything in phone is perfect i dont understand what the hell is going on, i have always done a complete cycle of battery since i bought it, i have always drained the battery completely till the phone gets off, and then i charge it in one go. in my other 2 iphones which is 6 plus 128gb and 7 plus 256 gb which i am using with this iphone dont have any problem like temperature etc etc. please help me with this mess