Looks like an awesome project! You will have either 3S 4400 mAH or 6S 2200 mAH depending on if placed in parallel or series, voltage and capacity both don't double when you connect batteries together. I also suspect the battery hatch will come open in flight unless some sort of restraint is installed as the internal structure will likely be pressurized from prop and motion. Even if initial friction fit is enough, that will go away after a few battery swaps...maybe some RE magnets or a servo arm latch like FT did on their new wing.