Hi I made battery modification on my Fc40. First I made batterytray under Phantom fuselage, material I use, is 2mm acrylic. 4000mah Gens Ace battery weights 320 gr. Two 2200mah battery weights 400 gr. I have gimlbal and Gopro Hero3+ installed. With this 4000mah battery, C.G is perfect. I have also 9" Vision props. On my first test flight, start rain, so I had to land, before first level protction.