Might be worth checking the USB port for power delivery - at least on the Tello it can be used to power payload.....I wonder if there is a convenient way to tap off power leads to an external payload?
The 3.5A cells may be hi voltage cells (>=4.35V instead of 4.21). You must get a usually cells, with 4.21V per cell:Yes you can . I'm working on adding additional batteries now... I've even replaced the 2500mah 18650 cells with 3500mah 18650cells. However no flight time was increased because the batteries I used were only rated for 10amps not 20amps... Looks like the best we can do for 20amps is 3000mah. That will at least put us in the 30+min flight time mark. Then add 2 3000mah 18650s on top, tapped into these solder points and we should be good for 45min +