DIY a High Capacity Battery for Helicopter

I have a LoGo 500 rc helicopter, I want to improve my LoGo 500 flight time. I according my LoGo 500 parameter to make some changes for it. Here are some parameters about LoGo 500.

Its main advantages are:
Considerably more transmission efficiency Very smooth running Extremely low noise level 155 teeth (mod. 0.7) Features: Constant tail drive Fully ball-raced Hollow 10 mm main rotor shaft 8 mm spindle shaft Carbon tail boom support Thrustbearing set for tail rotor with steel hub 3D vertical and horizontal fins Alu swashplate Light 3D paddles Standard-size and midi-size servos can be used CG can be determined exactly by adjusting battery position Specifications: Rotor diameter: 1150 to 1190 mm Rotor blades: 500mm/520mm/550mm Gear ratio main rotor: 8.1 to 12.75; Mod. 0.7 Weight: 2.6 kg and up Battery: 6S Lipo cells 4000 mAh.

Most parts I can't do , so I just use two 3s lipo batterie to instead a 6s lipo battery. My 3s lipo batteries capacity are 5000mah, so and two 3s lipo batteries in series are lighter and smaller than the 6s lipo battery, and I put them on my LoGo 500, and I test it, it can fly about 15 mins, it is successful.