Hello Darren,
Funnily enough I am having the same problem – exactly. The times are the same.
My situation is that my Phantom FC40 4000 mAh batteries are now a year old, and over winter I stored them at 11.4 v which my charger recommends as the best storage regime for Li-Po’s.
Reading elsewhere, Li-Po batteries need renewing every one to two years as they degrade naturally. I therefore presume that the batteries need renewing annually to get the 16 to 18 minutes flight time.
As an adage :-
My FC40 batteries are £21 ($30) and the P3 & P4 batteries are £108 ($156). Renewing the intelligent family of batteries annually is thus going to become expensive – far too expensive to consider upgrading to the P3 or P4. I have read that P3 owners are only getting 60 - 100 flights out of the intelligent batteries. Therefore, it makes sense financially to have something like the F550 where the batteries don’t cost as much.
How true the above is, I cant comment. I am no expert but its common sense that ALL batteries degrade over time and this is confirmed by what other people have said.