I have an Arduino Nano that will be powered from a 9V battery.
Here is the question , will data still pass through USB without causing problems for my laptop ?
I think it should be okay I think it will be okay and USB communication will work.From Arduino nano schematic I can see a diode that will keep the regulator's 5V from backfeeding into the PC.
Can I connect the battery power and use USB communication?
For your specific question, if you power the device via 9V battery, and connect this to your (powered-on) laptop, it should work fine and not cause any problems to your laptop. The diode indeed prevents (most of) the backfeed from battery towards laptop.
However, as always with hobby grade devices (and my comments in parentheses above), things may not be so well designed.
The first issue is if the laptop is off. It will not provide 5V out but the FTDI chip is powered. The FTDI chip circuitry is drawn in a way that it assumes being bus-powered from the computer, and does not implement being self-powered and sensing from the USB VBUS if the PC is actually turned on or even connected or not. The chip is always enabled and pulling one of the data wires up as a reponse to sensing power from the PC. But the PC is off. So this violates USB specification.
Another issue is the use of Schottky diode from the USB VBUS to 5V line. Schottky diodes are quite leaky in reverse direction compared to normal diodes, even if Schottkys have smaller forward voltage. So in the case of the circuit being able to sense if computer is turned on or even connected, the schottky might leak current in reverse so that it could detect a computer being connected even if unplugged. Likely if a turned-off computer is connected, it will sink the leakage current. Or not, and FTDI keeps pulling data line up, which again gets clamped by IO pin protection diodes of USB chipset.
The 5V regulator UA78M05 also warns about biasing the output above input supply. Deoending what there is on regulator VIN, plugging in the USB cable can push current in reverse through regulator to input of some unpowered power supply. Even if the internals of the regulator draws a diode, it may not be an issue as there cannot be more than about 4.7V over the regulator in reverse.
So if you are asking if it is safe or recommended to plug the Arduino into your most expensive Macbook or whatever computer, I recommend to always use a spare junk computer for electronics things, where a slight mishap may burn an USB port or the whole computer. Even at work, always plug an expendable old PC first, if it is not possible to measure the circuit to be safe to plug your actual work computer.
