I am using a 12S setup on the E5 with 556mm configuration, with a RX pack and I have been real happy with the Optipower 850 mAh 30C packs, previously I was using a little larger 1350 mAh pack. Optipower just came out with a 1450 mAh that I can't wait to get my hands on. But, the 850 gives me 3 good flights with the 3rd one being around 7.5 reading on the Vbar Control, so I could push one more if I wanted to (this is with Torq servos). Also, my 12S config is 2 2700 mAh 6S 50C Optipower packs that I heat shrunk into a stick pack.