ok, here is all i know for now.
1, check current draw of your retract servos. One i using drawn about 3-5Amps !!! look at this video (i wish, i seen it before i used same BEC for APM, receiver and retract servos)
https://www.youtube.com/watch?v=xjKB_i7S0UA
2, i do use two 5800mAh 6 cells
3, there was no low battery warning, because it was BEC failure caused over drawing by servo motors. I'm using 5 Amp BEC.
4, Yes, its my first time and will newer make that mistake again !!!
5, i was flying without retracts no problem day before and this happen on second take off. You can see on video, it happened right after i lowered retracts. It looks like there was high current draw in cut off time.
So, summary after crash:
broken boom, broken top plate holding batteries, APM, receiver and GPS, broken both retract servos (legs survived) and scar on my heart
My setup: Frame T1000 ironman, turnegy multistar 5008 340kv,1655 carbon fiber props, turnegy multistar OPTO 45Amp, 2x5800 6 cell, APM pixhawk, FPV 5.6 for pilot and Fatshark for camera man, DIY gimbal, retract landing gear.