Hello! I'm new to the DIYDrones community. I can't get my Pixhawk to arm unless it is connected to USB. From what I've read, this is usually a power related problem. The battery's voltage is 12.6 V (3300 mAh). from the power module it sends 5.27 V to the Pixhawk and 12.5 V to the ESCs. Mission Planner shows the voltage as well (12.6 V). 

My build is:

Pixhawk Firmware APM:Copter V3.3.3 Quad

DJI Flamewheel F450 ARF

Devo e7 + Devo RX701 + PPM Encoder

I've made it fly a few inches off the ground while connected to USB, but when not connected to USB, it simply won't arm. I've done the entire Initial Setup, flight mode is "Stabilize".

I read somewhere online that the Pixhawk comes with two cables that look the same but are not identical (power and GPS), I've tried with both, and both cables look the same to me. 

I don't know what else to check or what can be going wrong. Any ideas? What else can I check?

Thanks in advance!