[Andy A.]

There is no rule that specifically says your battery must be concealed (or, not one that I could find).

However, the forces involved when two robots collide are probably high enough to damage or crack the battery case. This would be a very hazardous and bad situation.

For that reason, I would strongly advise you to place your battery in such a way as if another robot runs into you, it would be difficult/impossible for it to make direct contact with your battery. It is not necessary to have the battery in a 'battery box', simply that it is not exposed to the playing field.

-Andy A.