Using Second battery to Charge Tanks

[wilsonmw04]

Originally Posted by jwfoss

Use battery one to charge the robot up, making sure that it is the robot controlling the compressor, not just hard wired or an external switch.

Turn robot off.

Remove battery and replace with a fresh battery two.

Go to field and compete.

I understand that's how it's done, but is it legal?

A robot is one compressor and 1 battery. By using two batteries you are effectively using more "electrical energy" than is available in one battery. This is what I am questioning.
When I was asking about off board compression, I was told we had to use the same battery as we would use in the match. Why is keeping the compressor on the robot given an exemption to that rule?