EDIT : Seeing the pictures this big, I think I know why and it is my fault. I tried to save energy by desoldering the led on the arduino. I followed a post here on mysensors.org. But I just realized I might have desoldered the wrong thing on this arduino. I'm not sure what I desoldered, but it might be what is causing the problems (I desoldered the tiny thing below the led) I'll put a fresh one to see if this is working.
Thanks
Original response :
Ok, thank for all the answers and here is more information.
I power it via 2AA battery. I did put a jumper on the battery point. And I am using a pro mini 3.3v (Cheap chinese clone). I am using a step up regulator to step it up to 3.3v for the arduino.
I measured the voltages and they are as follow :
Without USB programmer (Not working) :
Battery : 3.00V
Input step up : 3.00V
Output Step UP : 3.35V
VCC and GND on arduino : 3.34V
VCC and GND on Radio : 3.01V
With the USB programmer pluggued in (Working) :
Battery : 3.00V
Input step up : 3.00V
Output Step UP : 3.47V
VCC and GND on arduino : 3.48V
VCC and GND on Radio : 3.00V
Another information, I did not solder directly the arduino pro mini (you will see on the pictures). I don't know what the are called but I'll call them "extender". I use them so I can unplug the arduino in case I fried the pro mini.
Here are some pictures with and without the usb programmer plugged in :