Hi i wonder if anyone can help.  I have an iPhone  6 plus to which has no faults with the battery at all except recenty when connected to my p4 after 10 mins flight on a 70% phone battery it suddenly says low battery, then extra low battery then suddenly the phone will switch off, this to me is such a serious error.  So i have just purchase a dji mavic, great nice piece of kit, 7 mins flight flattened my iphone battery and it turned off. O M G Whats happening???.
  Strange thing is when i close everything down and switch the phone back on the battery is not fully discharged and is approx 10-12% left and my phone never goes off unless it gets to 1%.   Please help as this is a serious falt somewhere and im pretty sure its not the phone.

Could be your phone is not dropping linear and tanking too quick.  Li-po's seem to tank pretty quick when they go bad and your phone is almost 3 years old (Intro. Sept. 2014) and might be time to fade out.  DJI only warranties their Li-po's for 6 months.  Three years on a phone is rough, and DJI GO is very taxing and Autopilot even hints about it with their own app.  My 2 year old Samsung needs a charge everyday even if I don't use it much as a lot of Samsung stuff is running 24/7.  Seems worse over the past few months too.

Try a 100% charge on both devices and see if it gets through a 20 minute flight.  If it doesn't make it, might find one of those phone repair places for a new battery or time for a new phone.