I asked this question ten years ago.

The response was, a Battalion could be two or more companies, less than the standard ten company regiment.

In my ggrandfather's 43rd Tenn., CSA, they used the term to signify
five companies, half regiment, left or right wing of the regiment.

There may have been cases in which less than ten companies were formed,
thus, the designation as a batallion.

George Martin