I recently purchased a used Phantom 1 from someone that was upgrading. It came set up with a mount for dual batteries along with 7 batteries. Two of the batteries were the larger 2500 series. So i decided to see how much having these helped with the flight time. I started with the 2 larger batteries mounted. The phantom was heavy and although it took off, it flew like a loaded truck. And imagine my surprise when after only 7 minutes it signaled that it needed to land. Then I mounted two of the regular 2200 batteries. A little better in both flight performance and flight time. But still only 12 minutes. Then I took the external mounts off and just put one 2200 battery inside like it was designed for. It flew much better, sort of like a hot rod. And the flight time extended to a full 15 minutes before it showed the red light to land. I checked all these batteries for full charge before flight. However an interesting side effect was that the big batteries still showed 60% charge when the flight was over. The smaller units showed 40% and the single battery ran all the way down to 14%. There is something going on there having to do with the extra weight and drawing more amperage to compensate. So short answer, it looks like running just the one original battery works best for the little guy.