External Power = ok , USB Cable = Low Voltage out , USB Transfer is OK

I have this ODD problem.

If i plug a 7.4 volt Battery for supplying power to arduino all work flawlessly, If i plug the usb of my pc arduino blink the red led 7 times and dont run the sketch properly. But the USB Transfer with pc works fine.

I tryed to change usb cable, change pc, and to plug power directly from the usb jack, i'v plugghed the red and black usb wire directly to a 7.4 volt battery, and as well it works.

I mesured the voltage out in the two cases:

7.4v battery,

5.4v on vin 3.4v on 3.3v pin 5.4v on 5v pin

PC USB 3.4v on vin 3.4v on 3.3v pin 3.4v on 5v pin

I tryed to plug both USB cable and a 3.7v battery, but the result is the same. Red blinking led and not enough voltage output

So, is really odd...any suggestions?

nevermind -.- second arduino fried...

rx tx blinking... two times and no chance to make it talk with pc

If USB Voltage is low, then check the voltage drop of the Poly Fuse (gold thing near the USB connector.) If it is dropping any voltage, that means you are drawing at least 500mA from USB.