You know me well enough by now to know that I’m not writing about an armed engagement between two or more armies such as at Crécy, Poitiers and Agincourt. It’s still a military word, though and means a unit of men within an army, so battles did fight in battles.
Battles could be of different sizes, although they each were made up of ten banners. A banner was a flag which identified a man, a banneret, who could take men into battle with him under his command. I’ve written about these here. The men below a banneret would include a number of knights, esquires and other men-at-arms, often mounted archers in the fourteenth century. These could number up to sixty men, making a battle potentially six hundred men.
Sources:
A Dictionary of Medieval Terms and Phrases – Christopher Corèdon and Ann Williams
A Writer's Perspective 