The battle captain in the US Army is the "Watch Officer" in USMC speak. He is the "floor manager" in the battalion Headquarters/TOC/CoC, charged with monitoring all current operations, syncronizing assets, and notifying the chain of command of all developments.

Although normally called "battle captain", I have seen senior NCO's, warrants, lieutenants, and captains fill the role at BN level, and at BDE it is almost always a CPT or a Major. The marines use a Major or a senior warrant as the "Senior Watch Officer" (SWO) from my experience, and the Army is moving that way given all the assets managed by a BCT in OIF. Hence many BDE CDR's (such as mine) are placing post-command commanders in the role instead of the normal junior captains.