SOLVED : ROMEO BLE MINI coding problem

Hi everyone, sorry for my english

I'm using a Romeo BLE mini board (SKU:DFR0351) and a MPU-6050 (accelerometer + gyro + temperature sensor) built on a GY-521 board, both inside a spherical robot toy. I want the arduino to execute some code (actionning the motors) when the rotation speed exceeds a specific value. The sensor is linked through the i2C interface.

I use this sketch :
('', '
#include <Wire.h>

#include <Romeo_m.h>
const int MPU_addr=0x68; // I2C address of the MPU-6050
int16_t AcX,AcY,Tmp,AcZ,GyX,GyY,GyZ;
void setup(void)
{
Wire.begin();
Wire.beginTransmission(MPU_addr);
Wire.write(0x6B); // PWR_MGMT_1 register
Wire.write(0); // set to zero (wakes up the MPU-6050)
Wire.endTransmission(true);
Romeo_m.Initialise();
Serial.begin(9600); //Set Serial Baud
}
void loop(void)
{
Wire.beginTransmission(MPU_addr);
Wire.write(0x3B); // starting with register 0x3B (ACCEL_XOUT_H)
Wire.endTransmission(false);
Wire.requestFrom(MPU_addr,14,true); // request a total of 14 registers
AcX=Wire.read()<<8|Wire.read(); // 0x3B (ACCEL_XOUT_H) & 0x3C (ACCEL_XOUT_L)
AcY=Wire.read()<<8|Wire.read(); // 0x3D (ACCEL_YOUT_H) & 0x3E (ACCEL_YOUT_L)
AcZ=Wire.read()<<8|Wire.read(); // 0x3F (ACCEL_ZOUT_H) & 0x40 (ACCEL_ZOUT_L)
Tmp=Wire.read()<<8|Wire.read(); // 0x41 (TEMP_OUT_H) & 0x42 (TEMP_OUT_L)
GyX=Wire.read()<<8|Wire.read(); // 0x43 (GYRO_XOUT_H) & 0x44 (GYRO_XOUT_L)
GyY=Wire.read()<<8|Wire.read(); // 0x45 (GYRO_YOUT_H) & 0x46 (GYRO_YOUT_L)
GyZ=Wire.read()<<8|Wire.read(); // 0x47 (GYRO_ZOUT_H) & 0x48 (GYRO_ZOUT_L)
Serial.print(" GyX = "); Serial.print(GyX/131);
Serial.print(" | GyY = "); Serial.print(GyY/131);
Serial.print(" | GyZ = "); Serial.println(GyZ/131);
delay(333);

if((GyX/131) < -100 || (GyY/131) < -100 || (GyZ/131) < -100)
{
Romeo_m.motorControl(Forward,200,Forward,200);
delay(1000);
Romeo_m.motorStop();
delay(2000);
};

}[/code]

My problem is that this sketch works fine only when the board is running on batteries AND linked to the PC through USB.
When I try to use it only with batteries, it does'nt seem to work : the motors don't "move". I'm wondering why.

This original sketch from the product wiki works fine, when using it with the same batteries :
[code]#include <Romeo_m.h>
void setup(void)
{
Romeo_m.Initialise();
Serial.begin(115200); //Set Serial Baud
}
void loop(void)
{
char val;
if(Serial.available()>0)
{
val = Serial.read();
}
switch(val){
case 'a'://Go forward
Romeo_m.motorControl(Forward,200,Forward,200);
break;
case'b'://Go back
Romeo_m.motorControl(Reverse,100,Reverse,100);
break;
case'c'://Turn left
Romeo_m.motorControl(Forward,100,Reverse,100);
break;
case'd'://Turn right
Romeo_m.motorControl(Reverse,200,Forward,100);
break;
case'e'://Stop
Romeo_m.motorStop();
break;
default: break;
}
}[/code]')

Thanks for reading