Power supply best practice, when using a GSM/GPRS shield.

When using a common SIM900 GSM/GPRS shield with a Uno, is it best to have a separate power supply for the GSM and Uno?
Or as I have found, when the external power supply is plugged in the Arduino only, it also seems to power the GSM shield and operate properly.

Is this an acceptable configuration with one power supply? I'm curious about the current rating of the Uno in passing power to the shield. In the mean time, I'm checking schematics for the answer.

This matters since I'd like to run this prototype on a single 7.2V NiMh battery for a few hours at a time.
I suppose I could also create a two plug splitter for the battery if necessary.


SIM900 GSM module can draw upto 1A current in certain situations but the UNO board is designed to handle upto 500mA max current, so it is better to power up the gsm shiels with an external 9V or 12V DC with >=1A current o/p adapter (SIM900 has 5V DC regulator inbuilt, so it will step down the voltage as required)