[MrForbes]

Read the rules carefully. As I understand it, if you are using compressed air in your robot with no onboard compressor, then you have to power the compressor with the robot (not only do you need to use the robot battery, but you also need to control it with the cRio and pressure switch, and add another pressure release valve).

If you have a compressor on your robot, but you want to use an external compressor to pre charge it instead of running the robot to do this, then you probably ought to ask in the Q&A if it's legal to do so. The rules seem to imply that all the stored energy must come from the robot battery.

If you want a compressor on your cart for other reasons, you can probably do whatever you want as long as it's safe.