Back powering through USB. Dead arduino. Trying to understand why.

I am currently working a robot project that is using an Arduino mega2560 in conjunction with a raspberry pi. The Arduino is connected to the raspberry through the USB. I am powering the whole system with a 14.4V LiPo battery. I had the 14.4V going on the Arduino Vin pin and had the raspberry powered on the a 5V wall adapter. However the raspberry apparently is back-powered through the USB, that means that even with the wall adapter unplugged the raspberry is still on, drawing its power from the Arduino USB. The first time I turned this setup on, the Arduino's 5V regulator overheated and had a thermal shutdown. After this I have not been able to detect the Arduino's on my computer.

Currently this is the state of the Arduino: The On LED still lights up. 5V and 3.3V Still have the correct voltages when powered trough the USB. I am 95% sure that the program on the arduino was the Analog Write Serial Example, but neither Rx nor Tx LEDs light up when the arduino is on.

I believe that the FTDI chip or something concerning the arduino's USB was damaged, howerver I am not sure why. Is it safe to back power through the arduinos USB? I also had the raspberry plugged on the 5V wall adapter, could that have caused a short? Should I be safe repeating the setup on another arduino but not having the raspbery plugged on the wall, and trusting the arduino back-power as the only source for the raspberry?

Is it safe to back power through the arduinos USB? I also had the raspberry plugged on the 5V wall adapter, could that have caused a short? Should I be safe repeating the setup on another arduino but not having the raspbery plugged on the wall, and trusting the arduino back-power as the only source for the raspberry?

You answered your own question if you think about it. DO NOT repeat your experiment unless you have a large supply of surplus arduinos.

The correct answer connects your 14.4V LiPo to a switching buck converter (cheap on eBay). You then pass the 5v output to both the PI and the Arduino. I am already doing this with my robots. I assume the 14.4V power is for hi torque motors.

Certainly the Pi draws much more current than the Arduino regulator can be expected to handle, but it apparently shutdown which should have been the end of it.

I'd be very wary of using such a high-current power source with the Arduino directly, one accidental short and a portion of the board turns to vapour/flame... The DC-DC converter approach mentioned above is good.

Personally I've never left an Arduino connected to a powered-down USB host, it does sound a risky practice.