I've just gotten through drafting a battle scene a few minutes ago... I've had several of them in the series thus far. Like Brian says they are just like any other scene, just make sure you stay with character experience, rather than go for the scope of epic battle (I write in close 3rd, so can't speak for any other POV use really, maybe epic sweeping overviews are the way you want to write them)
But Every battle is epic for the one person we should be caring the most about. Maybe it's very cool to know that on the other side of the battlements some draws has single handedly lifted a catapult into a crowd of orcs at the bottom while dragons circle above him, but if our POV is got even 2-3 of his own orcs swinging swords at his his head then he's not going to care about the dragons or dwarf's strength.
Kerry gives great advice I think, focus on the speed, the sons the fear, the confusion. I know what a battle looks like, I can well imagine rank upon rank of warriors waiting for the order to charge and what not, but I have no idea about how it feels to be staring that down. Those are the details that we crave as readers I think (there woud be some to disagree with me though I'm sure).
And don't be afraid to miss things out. Not getting bogged down on the intricate details keeps pace and tension. I dont really care for the exactly how he put one foot behind the other and spun 180 while striking for the other guy's feet with his six foot oaken staff before stopping the over head swing with his iron gauntlet. I'd much refer something like, Spinning, he swiped Norman's feet from under him, and dodged back from Fred's strike. Not the best example, but you get the gist. That's my preference for fight scenes anyway.