I'm building my first drone and I've run into a problem I'm trying to understand. I use a cheap Diatone Mini 2A BEC V2.0 which gives me a steady 5.27v out of a 11.1v Lipo.

All components work fine with it but when I connect it to my flight controller the voltage drops near 0v when measured at any point on the flight controller and fluctuating between 0v and 1v when measured at the output of the BEC.

What could cause this voltage drop to happen? I measured all grounding points but these are ok (<5ohm). (and when connected through the onboard usb everything works fine)

 

Check your + battery in pin for short to ground.